Core concepts
Modules and readiness
Eight products you buy separately, and how each one tells you what it needs from you.
Bought separately, composed if you have them
Each of the eight is sold on its own and works on its own. They aren't stages of one product, and there's no order you have to buy them in.
Where you hold more than one, they compose: a fact one module owns is read by another rather than asked of you twice. Where you hold one, you supply the facts a sibling would have provided, and you attest to them.
You're asked for the fact, never for the module
A module never asks whether you bought its neighbour. It asks whether the fact it needs is present and good enough, and it doesn't care where the fact came from.
So running one module alone isn't a lesser mode. The same check runs either way. What differs is who supplied the fact, and that's recorded on the fact.
Readiness
Every module publishes a readiness report at /{module}/readiness. It lists each fact the module needs, who owns it, whether you hold it, where it came from and, when you don't, why not.
Read it before you start, rather than finding the gap in a refusal. A report that only said ready or not ready wouldn't tell you what to bring.