From our help page on tagging:
A tag is a word or phrase that describes the topic of the question.
Tags are a means of connecting experts with questions they will be
able to answer by sorting questions into specific, well-defined
Every tag, this one being no exception, should be used when whatever topic is being described by that tag is core to the question being asked. This is because the purpose of a tag is to make questions having to do with that topic easier to find by experts and other seekers.
If we start adding tags when they are only tangentially related, then that tag becomes noisier and less useful to someone using that tag to find things about, for example, targeting.
See this other answer I wrote that touches on a similar question, with a bit more emphasis on how it relates to system tags specifically.
Specifically relating to these two questions:
Are there any spells that can damage a creature in another creatures stomach?, by my reading, could very well be phrased as "Are there any spells that can target a creature in another creature's stomach?". The querent is asking about spells damaging creatures specifically with the concern that targeting them will be difficult and even cites the targeting rules for it. So it seems an appropriate description of the topic of that question.
Can destroy water completely dehydrate a person? I would not have added the tag here, personally. This question is asking about a specific effect can be achieved by casting it on a human. It's not asking whether that person is targetable or not and doesn't seem to concern itself with targeting in any way. It can probably be safely removed.