In SN 22.79 we found definition of each aggregate (khandha), among of them there is definition of sankhara:
“And why, bhikkhus, do you call them volitional formations? ‘They construct the conditioned,’ bhikkhus, therefore they are called volitional formations. And what is the conditioned that they construct? They construct conditioned form as form; they construct conditioned feeling as feeling; they construct conditioned perception as perception; they construct conditioned volitional formations as volitional formations; they construct conditioned consciousness as consciousness. ‘They construct the conditioned,’ bhikkhus, therefore they are called volitional formations.
While all other aggregates are defined roughly the same as other suttas definition of five agregates (e.g. SN 22.56), the sankhara definition here is denoting to sankhara as conditioned phenomena, i.e all the five aggregates, including sankhara as aggregrate. It seems this definition is out of context for sankhara as one of the aggregates, it should be defined as six classes of volition as in SN 22.56 or something like that.
Why SN 22.79 defined sankhara as conditioned phenomena, but not as volitional formation as it should be? Is it a possiblity of expansion of sankhara definition as aggregate to cover all mental factors excluding feeling and perception (as in Abhidhamma definition of sankharakhandha)?