What is a SaaS application?

Two engineering problems define it. Tenancy is how one deployment keeps each customer's data separate and impossible to reach from another account, which has to be enforced at the data layer rather than trusted to the interface. Entitlements are how the plan someone pays for maps onto what the application lets them do, which needs to live in one place rather than being checked ad hoc wherever a feature happens to be used.

Billing is where estimates go wrong. Taking a first payment is straightforward; the rest is not — mid-cycle upgrades and prorating, failed cards and retries, cancellation and what happens to the data afterwards, tax by jurisdiction, and invoices someone's finance department will accept. It is worth treating subscription handling as its own piece of work rather than a checkout button at the end of the build.