For when you know you are reinventing the wheel, but are doing it anyways. Questions with this tag involve code that is already fully implemented (such as from a library).
It is advised that such questions be free from criticism such as:
Library X already implements this code better; use that instead.
However, if it appears that the author is unaware of a better implementation (thus not writing the code for learning/practice), then he/she should be informed of that. Such questions should not use this tag.