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It is my understanding that a process is a specific-instance of a routing protocol. The process-id is a symbolic representation of a unique instance of that RP within a router.
If that's the case, what configurable/non-configurable changes can make one process stand apart from another? (Why would there a need for many RP processes?)
While not necessarily being split into different processes, some devices can run multiple OSPF instances for different routing domains. Each of these domains is entirely separate and addressing across domains may be ambiguous.
A common use case is a routing service for a client with private addressing that may or may not collide with the addressing of other clients.
You can use different routing protocol instances if you want the router to receive routes from different sources without sharing the routes from one source to the other.
For instance, your router connects to one network on interface 1, and it connects to a different network on interface 2. By using a different process on each interface, the router can install routes it receives from neighbors on eack interface, but it will not share routes from the neighbor on interface 1 with the neighbor on interface 2, and vice versa. This can be useful if the network on interface 3 needs routes to both of the other networks, but you do not want the other networks to share routes with each other. This prevents your router from becoming a transit point.
There are other scenarios, but the one above is one of the more common scenarios.
And if you are separating routes with processes, for example keeping finance segmented, add md5 authentication so that you are sure they can't interoperate.
For that matter, always use authentication for routing processes. Otherwise, I'll get up to some mischief with: https://snapcraft.io/quagga-tino-arm64 On my Linux box.
Quagga is the routing code that Ubiquiti uses on their switches. HPE/Aruba also use Quagga and stared the Open Virtual Switch project, now part of the Linux Foundation, to share it.
