Help on procmailrc recipe

I'm new to this group so please bear with me if this question is so
simple.

The manual on procmailrc says the start of the recipe is ":".
How is the number that follows that is used. I've seen
sample recipes that use ":0" , ":1", etc, but I do not have any
idea how these numbers are determined.

Any explaination or pointers to good resources are appreciated.
Thanks in advanced.

-- 
ricardo roque                 ikebukuro,tokyo   
ricky(_at_)dynanet(_dot_)ad(_dot_)jp