The term Dhamma is used for all dispositions (saMkhatA) and the non-disposition (asaGkhatA). Whatever is subjected to causality is called dispositions or formations. Nibbāna the only thing that is not subject to causality is called non-disposition of non-formation. The ∂hAtukAthaprakaraNa defines the samkhatā dhammās in the following way.
“ßaMkhatA dhammA paJcahi khandhehi dvAdasAyatanehi aTThArasahi dhAtUhi saGgahIta.”
=Dispositions comprise with five aggregates, twelve faculties and eighteen elements.”
According to the same source, the nibbāna is the only non-formed Dhamma and it comprises with one faculty (dhammāyatana) and one element (dhammadhAtu).
“åsaGkhatadhamma no kenaci khandhehi ekenAyatanena ekAya dhAtuyA saGgahIta.”
=The non-formed Dhamma comprises no with any aggregate but with one faculty and one element.”