I am looking for some information about the permissions set in UDF.
What type of permissions does HR get access to by becoming an employees HR representative in the UDF? Additionally, if the HR field remains empty (NO_HR) in the UDF, will HR be able to get the same type of permissions from RBP?
When you assign an employee's HR, this person will get the permissions defined in the HR Permission Role. So, please check the HR Permission Role (RBP - Role Based Permissions), to understand what this person will be able to see and act on.
Concerning the second question, if NO_HR is showing, it means that any of the HRs from your company will be able to act on this employee. Only when you assign one HR, he/she can act on.
I hope this helps!