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One record per person — contract, documents, salary history, reporting line, lifecycle events — and the processes that read it: attendance, leave, payroll, onboarding, performance.
The record is the product. Everything else is a workflow over it, which is why an HRMS whose record is incomplete fails at every process simultaneously.
The terms are used interchangeably by vendors and mean roughly the same thing. HRIS leans towards the record, HCM towards talent and performance, HRMS towards operations including payroll.
Do not spend time on the distinction. Ask instead which of attendance, payroll, leave and compliance the product actually runs, because that is where they genuinely differ.
The labels overlap. Ask which processes it actually runs instead.
When you need to know what was true on a past date, or when access needs to differ by field.
Availability, cost rate, reporting line and team membership. Those are the joins that save the most work.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.