relevant definitions from dictionary.com:
'site' means 1. a place where a structure or group of structures was/is/is-to-be located 2. place or setting of something, as in 'website'
'sub-' 1. below.. 2. a subordinate, subdivision
[to which one can infer: 'sub-site' and the compound word 'subsite', a place below another... a website within a website, a subspace, section or subsection of a website hmm..]
'section' 1. one of several components, a piece 2. a subdivision.. 9. a segment...
'subspace' 1. a space that is contained in another space
'community' 1. a group of people.. 3a. similarity or identity: a community of itnerests 3b. Sharing, participation and felloship
In my conclusion, it seems that community refers to the people associated with a website or subsite (a group), whereas maybe 'subspace' is a term more fitting as a common language usage for 'subsite'.
Of course, I haven't had a chance to use 5.0 yet, so maybe this is not relevant.