Repository = the highest level entity.
All commits are "across" the repository.
You can have multiple repositories (on single or multiple locations). They each will start at commit 0 (Repository creation)
And will indeopendently increnment from there. (A commit to reposittory 1 will NOT be recorded/recognised etc within Repository 2).
Beneath Repository you can break down into your own High Level Qualifier (which "looks" like a Directory)
and further down into Lower Level Qualifiers (as deep as you like)
At some point you will shove in your Trunk/Tag/Branch structure (to manage Parallel developments, code branches and "Baselines") at a logical (and managable granularity).
svn doesn't enforce any of it ... so you could have Trunk/Branch/Tag at HLQ Level 1
or beneath any chosen level (per App, Per sub-app, per-technology) - its all up to you.
You can even mix and match (Trnk/BR/Tag at different levels within a single repository - Although I really wouldn't recomend this!!!