Full Idea
An aggregate, but not a composite substance, is resolved into parts. A composite substance only needs the coming together of parts, but is not essentially constituted by them, otherwise it would be an aggregate.
Gist of Idea
A composite substance is a mere aggregate if its essence is just its parts
Source
Gottfried Leibniz (Letters to Des Bosses [1715], 1716.05.29)
Book Reference
Leibniz,Gottfried: 'Philosophical Essays', ed/tr. Arlew,R /Garber,D [Hackett 1989], p.203
A Reaction
The point is that there is more to some things than there mere parts. Only some unifying principle, in addition to the mere parts, bestows a unity. Mereology is a limited activity if it has nothing to say about this issue.