What are the mrb and pal parameters and what are they used for?

mrb parameter
The Merchant Referral Bonus (MRB) program is an old Channel Partner bonus program that was used to identify and reward merchants for referring other merchants who signed up with PayPal.

<input type="hidden" name="mrb" value="XXXXXXXX">

pal parameter
The pal parameter can be either the encrypted account number or the merchant's email address.

<input type="hidden" name="pal" value="XXXXXXXX">

Although PayPal has discontinued this program, the variables were left in place for accounts that signed up while the program was still in use.