Base memory can participate in OnLine Memory Delete operations (it can only be removed from a nPar [Cell OLD] or a vPar [Memory or Cell OLD] when the partition is shut down or rebooted for reconfiguration).
Floating [also known as Ejectable memory] can be OnLine Deleted.
The reason to have Base memory is that most kernel allocations are not OLD'able, hence the kernel is restricted to Base memory. The partition should be configured with sufficient Base memory to provide for the kernel memory usage to support your workload. (The File Cache memory is still User memory, by the way - it is the system's cache of User pages and hence can use Floating memory).
