Original: Between 1992 and 2001, two different numbers were given to subtropical depressions or subtropical storms, one for public use, the other one for NRL and NHC reference. For example, Hurricane Karen in 2001 was initially known as Subtropical Storm One as well as AL1301 (or 13L for short).
Simplified: After that, subtropical storms were just numbered, such as Hurricane Karen from 2001 which was originally named Subtropical Storm One.