Even with class scopes and subpool partitioning, a variable is still vulnerable if multiple methods within the scope try to access it at the same time. To handle this, REXX rules that once a particular method is activated and exposes variables from its subpool, that method has exclusive use of the subpool until processing is through. Until then, REXX delays running any other method needing the same subpool.
Thus, within a single scope, if different activities send multiple messages to the same object, REXX forces the methods to run sequentially. This "first-in, first-out" processing of methods in a scope prevents them from simultaneously accessing any one variable, and maybe corrupting the data.