The tag is intended for questions that are intimately related to English.
For example, comparing an English word with a the Latin word it comes from is one way to use it.
Most of the questions on the site are in English but very few are about English.
The tag is for the latter kind.
Many questions use English only as a medium.
For example, many translation questions from Latin to English are more about understanding Latin than writing in English.
English is a convenient common language — not least because the interface is in English — but many translation questions would work equally well if English is replaced with German, provided that the OP is fluent enough in German.
As for many tags, the boundaries are not clear.
They could indeed be clarified with a good usage guidance.
Perhaps something like this?
This tag is for questions concerning the relations between English and Latin (or Greek). Questions have to be about English (to some extent), not necessarily in English.
Whether you like this or prefer some other description, I suggest going to the tag page and write your usage guidance.
It doesn't have to be perfect; others can improve on it later on, just like most other content on the site.
