I try to describe the difference between all three.
Process Discovery - rule to find automatically processes on monitored hosts. You can already here define default parameters that should be applied to the found processes.
Service monitoring rules → State and count of processes - only applies parameters to already found process checks. The matching condition here is a name of a discovered process. This discovery is done with discovery rule.
Enforced services → State and count of processes - you define process checks on hosts without discovery. This is useful if you expect a process to run on a system and don’t want to rely on the discovery feature.