Namespace management in TeX is a co-operative affair: we all share one space, so conventions such as
\my@clever@macro are important. For LaTeX2e work, this has always been done on a very informal basis: look around and find a space! For LaTeX3, it seems like a good idea to make things a little bit more ordered. We’ve therefore set up a simple flat-file prefix register, which will track all of the prefixes in use in expl3 code (provided people tell us, of course).