Platform Designer base address

Base address is editable and represents the offset in the address space for the host accessing this agent. It's not clear what host is connected to the debug_mem_slave agent in your screenshot, but whatever it is, it will start accessing this agent starting at address 0x10800 (instead of at 0x0). The end address is not editable. It is automatically determined based on the addressable space of the agent, so as you note, this is at address 0x10fff.

The tool automatically fills in a base address if you don't enter it manually. You can automate this to avoid overlapping address ranges using Assign Base Addresses from the System menu. If you want to use a combination of manual and automatically assigned addresses, set a base address, then click the little lock icon. The select Assign Base Addresses.