HR market is as big as FI/CO. Just think that every company has financial accounting and human resource needs, in every industry, in every country, in every company size.
The SAP Market in general, looks very good. Particularly SAP HR looks excellent. You will find lot of consultants with knowledge on PA (personnel Administration), Personnel Development etc.
If I were to get into SAP HCM, I would focus on Payroll. It is because Payroll is very complex and challenging and it has lot of dependencies on other components such as PA, OM, PD etc. So if you focus on Payroll, you would, for sure, learn PA & OM too.
OR the best thing to learn is Reporting. If you try to understand what each of SAP HR reports do, you will proabbly end up learning component by component. See also; HR Functional Specification Example
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