As Sankarsan said, at any time or for one person, one reason type is applicable.
Each reason type will lead to a different redundancy model. See under IMG->Payroll (of relevant country)->Terminations/Redundancies. There are several steps to configure - eg., Term'n reasons, Redundancy model, Redundancy w/t's, Redundancy rules for paying leave balances/accruals, etc.
For Terminations, with Public sector, different transaction code (and program) would apply in some countries like Australia.
User group dependency for any personnel action is to control which user group can use what type of pers.actions / infogroups. For example for HR admin users, only certain non-value infotypes should be allowed - e.g., IT 1, 2, 6, etc. For Payroll admin, IT 7, 8, 41 should be allowed. For SAP Support person all IT's should be allowed; however during investigation/testing he/she would set up user profile with UGR as applicable for the business area like HR, Payroll.