SR, CR, BUG, or if there is just a number are all changes the resulted from a bug found in the product.
US, EPIC, or RLS are all product enhancements that are added to the product.
The reason for the various naming conventions is due to the bug tracking system/process changing over the years. Some teams use different processes and that results in a different final naming conventions. The identifiers are more relevant for the Support team to track and to be able to review in Pega's internal systems when necessary.