You can assign as much memory to a logical domain as your applications need, as long as the amount does not exceed the available memory. The available memory is a function of the number and size of the installed DIMMs, the amount of memory in use by the system, and the state of DIMM sparing. See DIMM Sparing for more information about this feature.
The amount of memory being used by the system is noted as _sys_ memory in the ldm list-rsrc-group command output, and it is different for each server. All of the memory is assigned to the primary domain in the factory default configuration. You must remove memory from the primary domain to assign it to another domain. The primary domain should retain at least 16 to 64 gigabytes of memory.