Combination of two, three etc. Paramāṇus (the ultimate atoms);
Skandhas (material aggregates) of two Pradeśas (the undetachable indivisible units of the substance), three Pradeśas etc. illustrate Itaretarasamyoga.
duppabhitīṇa paramāṇūṇaṃ jo saṃjogo so itaretarasaṃjogo bhavati paramāṇūṇaṃ.
(U Cū p.17)