@@spid is the unique Id SQL server assigned to the connection, which in this case, would be executing your stored procedure.
if you run exec sp_who2 on your server, you'll see that most connections above 50 are various user connections; within that connection they may run multiple queries, but when someone disconnects, the spid context_info related to the spid would be destroyed, and the spid could then be assigned/reused for a new connection.
@@procid would be the id of the calling stored procedure, which is another option; you could do WHERE OBJECT_NAME(@@PROCID) = 'MyProc' i think, but that needs to be tested and verified...@@procid might return the id of the trigger instead!
Lowell