1.
creates a variable that resides inside a module and can be accessed by all actions inside the module. This variable can not be accessed by a another module. A call from another module would return nil. 

If you need to pull a self. variable from another module use the modulename.variablename  
-- loose the self. part in the calling of the variable 

Example:  

in SimpleUI create 

self.b 

in another module  

self.c = SimpleUI.b  

2.
Call an action inside the same module:

self.action2() 

triggers the action called action2 inside the same module.