"combinations" and "permutations" are ways of calculating how many different ways there are of doing a thing.
e.g. for pick 4 shards out of 15 and try to combine them (whether the order doesn't matter) the calculations is :

n! / ( k! * (n - k)! ) = 15! / ( 4! * 11! ) = 1356

anyway, we don't need to do this because all the shards have a root attached to them, so as long as that's in the forward message (which the schemas you made say it is) then we don't need to guess which shards go with which.... so it's just a matter of using all the shards you have then checking some MAC to see if you had a quorum