A business model is how a business produces something worth paying for, and how the money reaches it.
The parts
Who it serves — see target market.
What it delivers, and why that is worth choosing over the alternatives. This is the value proposition, and it is the same question as competitive positioning.
How it charges — per job, per hour, per month, per unit. See pricing models.
What it costs to deliver, which decides whether the rest works — see gross margin.
Why it is worth examining rather than assumed
Most service businesses inherit their model from the trade rather than choosing it: quote a job, do a job, invoice a job.
That model has a structural property worth noticing — revenue resets to zero every month, and every month starts from nothing.
The models a service business can move toward
Recurring, where a portion of revenue arrives without being re-won — see service agreements and building recurring revenue.
Higher value per job, serving fewer customers better — see increasing average job value.
Productised, where a defined package is sold repeatedly at a known cost rather than each job being estimated from scratch — see rate cards.
None requires a different trade. All change the economics of the same work.
Testing whether it holds together
Whether the price covers full cost including overhead and a return; whether enough customers exist within reach; whether the business can deliver at the volume the model needs; and whether it survives the owner stepping back.
A model that fails the last one is a job rather than a business — see key person risk.