procmail codes

To all:
  Where is there a list of all the procmail commands?  A few of them are 
in the man page procmail(1).  My server may not have everything.  It 
would be nice to understand the secret code.  

What does E mean?  example :0 E

What does "X-Loop:   do ?
   example | formail -rA "X-Loop: Processed_Mail"

Why is the A in the above example?

What does cat do?  Is it a series of commands or a command word?

I am sure that people new to procmail would like to know these types of 
things, so can we find out where all the experts learnt it from.

Thanks from all new procmail users,

Patty Buchanan