x1 is an (arbitrary) x2-set (li) of superset x3; x1 is subset/subgroup/subcategory/subclass/vel sim. of x3 with cardinality/size x2.
x1 is empty if(f) x2 = 0, which is possible; x1 may or may not be a proper substructure (praperi) of x3. x2 is a nonnegative cardinal. The two distinguishing features of x1 are its size (x2) and the object/structure (x3) to which it belongs/which contains it. Any x2 elements of x3 can belong to x1 as long as the total count is correct; no particular collection is necessarily included. It is bad form for x2 to strictly exceed the size/cardinality of x3 and, necessarily, no such object/structure can exist. See also: klesi, praperi, cletu.