Supernetting :-Joining two ot more networks to form a larger Network according to the requirement of user.
In supernetting , supernet bits are bits are borrowed from Network portion. I mean to say here you convert some of the network bits of 1's to 0's (from right).
Actually it's opposite of subnetting. subnetting is just dividing the network and create more subnetworks while supernetting is joining some networks and making one supernetwork.
If you have class B subnet mask as /10 then you know by default class B subnet mask is /16 but as here it's 10 so certainly some of the network bits (here 6 ) are converted into 0 and the goal is making supernetwork.
so you can guess if subnet mask is >16 for class B then subnetting done.
if its <16 then supernetting done.