System-Agnostic: The System Tag Meaning "Not System-Specific"
The system-agnostic tag is for questions where the asker is looking for answers that are not tied deeply to a specific game system's rules, usually for portability. It is asking in a context that is "agnostic" to the "system" being used. This is extremely straightforward; don't overthink it. It is just a system tag that says "not one specific system."
"I am asking from a general RPG standpoint, not from within the frame
of a single system."
As noted in those previous metas, people here love to urge someone to add system tags in nearly every scenario. "But what GAME SYSTEM are you using when you have demotivated players?!?" And that's fair; game systems vary widely and in this case, if someone is just playing, for example, D&D 5e, and they ask about demotivated players in a pure D&D 5e context, you might cite XP awards and Inspiration and advice from the DMG and so on.
However, some people (like me, which is why I am an advocate for the tag), play many, many different RPGs and we have some issues that we want solutions for that are more general. If I have demotivated players, I might want more broadly applicable techniques that will help regardless of the game we're playing that week. As a site, we support those kinds of players and that need.
You might have the "same question" asked with a system tag and with a system-agnostic tag and have them get different and good answers, just as you might ask the same question with two different system tags.
Because it is a specific declaration of the querent's intended scope of the question, then as @AceCalhoon points out, you should tend to leave it alone. You can ask the querent about their intent to clarify, but under no circumstances should you be trawling through old questions and changing either their system tag or system-agnostic tag unprovoked. That implies that you know better about what the querent needs than they do. Maybe, just maybe, someone is wrong about what system they're asking about (or asking about system-agnostic), but it's probably better an "ask them to clarify themselves" situation.
Q: Does this site require a system tag?
But in practice, this site does effectively require a system tag, one is on a huge percentage of questions and when it's omitted the question gets closed and/or a system is asked for a vast majority of the time. The chance you can ask a question without a system and not get hassled for a system is less than 1%.
Q: So, does every question without a system tag need system-agnostic?
I don't know, does it?
If people are willing for someone to ask a techniques or social question and not badger the querent into a system tag, then it's not needed on every question. But if people are asking for system clarity, and especially if they're closing the question because there isn't one, then it is a legitimate option (as is a system tag) to set a scope explicitly. If someone up front wants to declare "I know what I'm asking for and it's for system-agnostic", they can use the tag. We should respect that scope choice.
If you look at the most recent system-agnostic questions, and look at the highly voted open ones, many have people pressing them for a system in the comments and the querent wants to say "stop it, I don't want this specific to a given system." (See these questions, for example: How to Roleplay an Intimidation-Oriented Character Without Being a Net Negative for the Party, How can I show-not-tell my players that they are The Bad Guys?, How do I make quests rewarding for my player?, etc.)
Q: Can you use system-agnostic and a system tag on the same question?
That would be very uncommon, but could be legitimate based on the question. "How do I bring the competitive feeling that Amber Diceless Roleplay has into an arbitrary other RPG?" could have the amber-diceless and system-agnostic tags. If both are being used, the question should probably be explaining itself really well.
See also: Does the [system-agnostic] tag conflict with system specific tags?
Q: But is the system-agnostic tag really needed?
We've had 468 questions using the tag. It is the 21st most popular tag on all of RPG.SE. So yes. Many posters find it useful, so there's a pretty high bar to just wave hands at it to do away with it.
The highest-voted question on the site is tagged system-agnostic: How do I get my PCs to not be a bunch of murderous cretins?, where the tag was specifically used to try to get people to not obsess about D&D-specific answers.
As you can see it's a mix of player/GM techniques, problem player/GM issues, gaming style issues, campaign and adventure design questions, and so on. Which one could ask for a specific system, but for those who have a wider scope of RPGs, you want to ask it across them.
Q: But isn't this a "tag tax?"
All system tags are a tag tax, if you want to define it in that way. They set the scope of the question in a way that makes answers meaningful. If I give a FATE answer to a D&D question, it doesn't help the querent, and that's what we're here for. If I give a FATE mechanics question to a problem someone wants to solve across game systems, it is similarly unhelpful. system-agnostic is a system tag like any system tag, like "agnostic" or "atheist" or "non-denominational Christian" or "mainline Protestant" is a valid answer to "what religion is this question about?" In any event, these are useful scopes for questions and answers.
Q: But isn't system-agnostic a meta tag?
No. See this Q&A: Is [system-agnostic] a meta tag?
Q: Should I use a system tag, system-agnostic, or no tag on my question?
If you are playing a given system and you want answers for that system, add a system tag. Don't artificially widen it "because more people would find it helpful," the site works best when you ask about your specific problem.
If you are deliberately asking for cross-system techniques, put system-agnostic on it to shortcut people having to ask.
If you're asking a problem player question or something generic, you don't have to use either kind of tag, but you will likely be pressed to add a system tag – and you can then decide to add a system tag or a system-agnostic tag to answer people and head off the close votes.
If you are playing a freeform game, where there is a "system" but it's not one of the listed systems, you can use the freeform tag instead.
See also: Should I use a narrow system tag, or use a broader tag?
Q: Can I just assume that a question without a system tag is system-agnostic?
No, because "bad question" is another option. See this Q&A: Is it against policy or bad practice to attempt system agnostic answers to questions whose system has not been specified? – in that case, the answer was "you definitely should not have in this case." Alternatively, it may just be that the querent has failed to specify what game system they are playing; we have guidance for approaching such questions here: What should I do when a question does not specify the game system being used?
Q: Can I use system-specific examples when answering a system-agnostic question?
Of course. Keep in mind that if your answer is too dependent on one system, especially if that technique isn't easily portable out of the system, it may not be judged to be super helpful by the OP/community, but illustrating your points with specific systems is helpful. The querent will probably illustrate their question with examples from specific games/sessions too.
Q: Should I keep bothering someone who has declared their question system-agnostic to pick a system?
No, you should not; it's rude. If you think they are new and don't understand tags or something, you can give them a quick "Hey, if you are really just looking for an answer inside system X, then you might get better and more relevant answers if you pick that system", but then you should respect their choice. No one has to pick a specific system if that's not what they want to ask, and you should not try to make them do so.