Even with class scopes and subpools, an object variable (attribute) is vulnerable if several methods within the scope try to access it at the same time. To handle this, Rexx ensures that when a particular method is activated and exposes object variables (attributes) from its scope, that method has exclusive use of the scope variable (attribute) pool until processing is complete. Until then, Rexx delays the execution of any other method that needs the same scope object variables (attributes).
Thus if different activities send several messages to the same object, Rexx forces the methods to run sequentially within a single scope. This "first-in, first-out" processing of methods in a scope prevents them from simultaneously accessing one object variable (attribute), and possibly corrupting the data.