Update
This question was previously titled "What is the code tag?".
As it was before, my question wasn't super useful. Since it looks like there isn't necessarily an overwhelming rhyme or reason to the tag (see Paul's comment), the question should more appropriately be "what should be done about the code tag?".
Question as it was before
What is the code tag? It has no wiki summary, and I can't really seem to find any apparent pattern among all of the questions that have been tagged with code, at least not immediately.