What is the importance of "Set Activation date " in LIAM portal

From my understanding your are just deferring the change to that object to the date you set when clicking on set action date.

For example if I click on an employee in the Manager client and then assign them to an application role, once done instead of clicking on save I would click on object in the menu and then the option define execution time. If I set that to Monday September 3rd then this creates a deferred operation that will kick off on that date. You can see deferred operations within Manager by clicking on the menu Database and then Show deferred operations

i am referring to New user creation in One idm portal.While New User creation we have that option "set activation " date.Does it mean that when the activation date got reached ,the entire activation process will begin automatically.

i am referring to New user creation in One idm portal.While New User creation we have that option "set activation " date.Does it mean that when the activation date got reached ,the entire activation process will begin automatically.