Agents are not engineers
The word agent appears all over BMS and means something specific that is easy to confuse with something else.
An agent is a party in a commercial relationship. A finance company that funded a machine. A service business covering a territory you do not. They earn from activity you process, and the Agents area exists to work out what they are owed.
An engineer is a person who attends a job. Engineers live in Service, they are linked to user accounts, they carry a phone, and they fix machines.
Both appear on a service job — the job has a Service Agent and the activity has an assignee — and they are answering different questions. The agent is who the work belongs to; the engineer is who is doing it. An engineer can attend for more than one agent, which is only sensible once you see that the two are unrelated.
Why agents earn without doing the work
A finance agent never touches a machine. What they did was fund it, and the contract that pays for that machine each month owes them a share. Reconciliation is the process of finding those shares in a period's invoices and settling them.
That is why reconciliation types are described in terms of invoices within a period rather than jobs or visits: the trigger for an agent earning is that something was billed, not that something was done.
Why service agents are internal or external
A service agent might be part of your own business — a branch, a division — or a third party entirely. Both need the work attributing to them; only one of them needs paying externally for it.
Keeping that as a second classification, rather than as two separate kinds of agent, means the service side of the model stays one thing. A job is assigned to a service agent the same way regardless, and the internal/external distinction comes in later, at reconciliation, which is the only place it makes a difference.
Where the money goes next
An approved reconciliation results in purchase invoices in Stock — the business owes the agent — and those are pushed to Xero as accounts payable. So the chain runs: a contract is billed, the invoice names disbursements, a reconciliation gathers them, and the result becomes something payable in the books.