AnySubset

AnySubset[{c1,c2,…}] represents an element in an interpreter or form that accepts any subset of the choices ci. 

AnySubset[{lab1->c1,lab2->c2,…}] accepts any subset of the labi, yielding the corresponding ci as results.

AnySubset[EntityClass["type","class"]] accepts any subset of the entities in the specified entity class.

AnySubset[choices,max] allows at most max choices to be selected.

AnySubset[choices,{min,max}] allows at least min and at most max choices to be selected.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.