TODO indicates "task to be completed", FIXME to be fixed etc. Is there a code tag to indicate "need improvement" on code comments?
The closest thing to a document trying to standardize this is the Python PEP 350 - which was rejected as not a standard.
The Mnemonics section has a list of various ones used - I hesitate to call any of them "commonly used" beyond "TODO", "FIXME" and "XXX" (which are predefined in eclipse).
Going beyond this, it is local convention of that code base and team.
The new question - is there a "need improvement" type code tag. The best practice would to use the minimal set of tags. "TODO" indicates a task in the code.
// TODO this code needs to be refactored would be sufficient rather than using another
// RFE should refactor or
// IDEA you know, we should refactor this.
TODO is already recognized by Eclipse, by convention, by mylyn.