For questions about the formulation of a proof, not about the mathematics behind it.
Questions with the (proof-writing) tag are about the presentation of a mathematical proof. Questions might include:
- Should I include [x-mathematical detail] at [y-part of this proof]?
- Here is a proof about [x-mathematical idea]. How can it be improved?
- Is the following a sufficient proof of [x-mathematical tidbit]?
But this tag is not for asking someone else to write a proof for you, or for how to answer some question. Questions such as: My professor asked me to prove the Pythagorean theorem and I don't know how to begin are not to have this tag.
This tag is intended for use along with other, more "mathematical" tags. A question about the writing of a proof in abstract algebra, for example, should have the abstract-algebra tag as well.
See here for a useful set of guidelines for writing a solution.