Ticket
The unit of tracked work — a single logged customer request and everything attached to it.
A ticket is the atomic record support runs on: one customer issue, its conversation history, status, owner, and metadata. Nearly every downstream metric — volume, cost per contact, resolution time — is counted in tickets, which makes the ticket both the workhorse and the source of a lot of measurement mischief.
There is no formula; there is a definition, and that definition is where teams diverge.
What it hides: what counts as one ticket is a policy choice. Merging, splitting, and re-opening all reshape your numbers without any change in actual work. Two teams with identical workloads can report very different volumes purely because of how liberally they open and close tickets.