As in all things IBM JCL, to invoke a default you may omit parameters and sub parameters.
With the EXEC statement, the default is PROC= the system assumes you are executing a procedure rather than a program. This is a throwback to the 1960's (and unfortunately i still remember them) where all JCL was invoked using ACTUAL puched cards. Looking at JCL you will see many 00's of lines of code that had to be fed through a card reader each time the job was executed.
This was a nonsense situation so procedures were born, allowing the now 'card images' to be stored on DASD and invoked with //STEP1 EXEC MYPROC1. IBM assummed all users would want this so made PROC= the default