A session is a visit, appointment, treatment, or set of notes for a particular animal at a particular time.
For example, if you see Spot the dog on Tuesday for an adjustment, that's a session. If you see Spot twice a week for a month, that's 8 sessions.
You 'spend' a session the very first time you save it, all subsequent edits, saves, emails, invoices and everything else from that point forward is free. Sessions are not linked to log-ins, nor to the number of times you open or edit a set of notes. Just the first save, once. You can also enter as many animals and contacts as you like, that's free too.