The force and power limits are in general most relevant towards “clamping” situations.
Essentially, you are looking at the maximal pressure the end-effector or robot is able to excert on a human.
Hence, the force by area, so it also depends a lot on the design and shape of the end-effector.
The speed and momentum limits applicates most towards a “free-space collision” situation.
Here looking at what kinetic energy, a human body should have to absorb in such a case.
In general, it all comes down to your risk assessment, and compliance with national laws.
There is a page or two in ISO 10218-2 regarding this, but also there are some optional guidelines and good thoughts available in ISO / TS 15066 regarding setup of these limits.