As shown on https://superuser.com/questions/tagged/register

There are only 16 questions tagged with [tag:register].

Within them, I found the following "meanings" for register:

 - register as Registry
 - CPU registers
 - Registering software
 - DLL registration

Noting the lack of a proper definition in the tag info page and the number of questions actually using the tag, I don't see any reason why it should exist as is.