Well known SIDs are, eh, well known and are the same on all machines, other SIDs have a randomly generated machine/domain part and the number after the last dash is called a RID and represents a group or a user on the machine/domain. The filesystem does not care, it is treated as a normal name but the folders in the recyclebin have the ACL set to allow access for only that user. The recyclebin is a shell concept, the low-level file APIs in kernel32 treats it as any normal folder tree.