Difference between Group A & B

hello all,
I am getting confused between the restriction group types A & B. I tried to experiment this by:
 

So now , as per the above screenshots the system would now restriction the user Pam Brawner to have access to the selected 2 subaccounts.

Now when I created a new group type B:
for the same user
 

So now if I would select the 2nd Subaccount, would that mean that now the system wont show the subaccounts selected under both Groups A & B for the same user or what? I am getting confused here. Would like your guidance in this.
Thanks in advance.