What is the difference between "time_closure" and "present" options in NIC-400?

Hello,

I am trying to add a register slice to an AXI ASIB in my NIC-400 configuration using the uArchitecture Editor in Socrates.  The Timing Closure tab gives me the options of "time_closure" and "present" for each AXI channel.  What is the difference between these two options?

Thanks,

Lloyd