Ambiguity refers to words, phrases, or sentences which can be validly understood to mean multiple different things

What is ambiguity?

Take a look at the following sentence

They saw the tree

Does this mean that in the past, they had seen the tree, or does it mean that right now, they are sawing the tree? This is an example of semantic ambiguity, where a word can mean multiple things under that context.

Take a look at this sentence

Adam sent the essay to James to help him with his writing

Do him and his refer to Adam or James? This is an example of syntactic ambiguity, where the positioning of certain words allows for the sentence structure to be interpreted in two ways giving two different meanings.

Where should the tag be used?

The ambiguity tag should be used whenever a question involves anything to do with ambiguity, including requests for explanations on what the true meaning was, requests for suggestions on how to avoid/resolve ambiguous constructions, or other similar subjects.