This answer has copies the text of Zxyrra's question (see above) for easy reference. It has been italicized for clarity. Apologies for any formatting problems
According to its tag wiki, xenobiology is
*a form of biology that is not familiar to science and is not found in Earthly life*
the wiki and its contrast with the following imply that it is about aliens.
And biology is
*For questions about living things. Does not exclude aliens, but additional information is usually necessary*
I believe there is a significant overlap between, and confusion with, these tags.
*Not all aliens are xenobiological. Most questions involving alien organisms are not "not familiar to science". They may ask about evolution, or anatomy, or some other well-understood concept, but it is rare to find something completely unheard of. Therefore, most "alien biology" is not "xenobiology" by the tag definition, even though the tag is implicitly about aliens.*
*Not all xenobiology is alien. Some of the questions that do describe processes "not familiar to science" are eligible for "xenobiology" by definition, but they are not about aliens, which the tag implies.*
Why do we need overlap and ambiguity? I'm suggesting this:
There is no harm in overlap and some ambiguity. This is inevitable with any classification system like WB SE's tag system. The problems here are trivial compared to real classification systems. Besides xenobiology can be specifically about alien biology, despite alien biology being dealt with under Biology.
One further point of necessary clarification. Xenobiology can dealt with two fields of biology (1) alien biology concerned lifeforms beyond planet Earth, (2) strange, bizarre, and weird biology involving organisms that can be Earth-based. The tag as currently written, vaguely wanders between the two fields and could be a source of confusion. Though, probably, only to persons who have some knowledge and understanding of the relevant biology.
biology should for questions about all living things, fictional, real, alien, Earthly, etc. because it is the study of life. Alien life is still life.
xenobiology should be merged with biology or synchronized. Its usage is not clear, as explained above, but all questions with this tag do discuss the study of life or aspects of life.
creature-design is relevant if the creatures are truly alien, and their peculiarities can be tagged separately.
Agreed. A good idea.
or alternatively to the above "xenobiology" can strictly be about "aliens" because "processes not known to science" can happen on Earth, too.
This is a good point. It makes sense here on WB SE to strictly corral "xenobiology" to alien biology. Although this means questions about strange, bizarre, and weird biology will have to be dealt with under "biology". This shouldn't be a problem as these questions are likely to be rare. I may be wrong, but if so, then consideration will have to be given about using "alien biology' as a tag and restricting "xenobiology" tag to weird biology.
Having read the text of the current "xenobiology" tag, it does need editing, if for no other reason than to expunge one awful spelling error, but the "hard-science" requirement (mostly overlooked) is unnecessary hinderance. "science-based" would be better. The current "biology" tag is not unreasonable, and may only need minor edits.
There was a suggestion about using "alternate-biology" (for reasons I won't go into here, "alternative-biology" is to be preferred). This makes better if used as a tag for "alternative-biology" in "alternative-universes" or "parallel-universes" (or worlds for universes) in the same sense as "alternative-physics" in "alternative-universes" or "alternative-cosmos"
Note: other terms that could be used as tags for alien biology (apart from "alien biology' itself) include exobiology and astrobiology. "Exobiology" has mainly fallen out of usage, so may be ignored safely. While "astrobiology" is in vogue in the scientific community.