A work process runs in Priv Mode, when the transaction currently processed allocates more than

em/address_space_MB - em/global_area_MB

of memory. This portion of an ABAPs memory is shared memory and can be accessed by every work process.

The default values for these two parameters are depending on the platform (Windows 32 Bit 512 MB, Window 64 Bit 4 GB).

If the transactions needs more memory it is allocated as Heap memory in this process. As Heap Memory is not shared Memory the transaction cannot be rolled out and rescheduled on a different work process when the user is inactive.

it stays on the work process it is currently working on which therefore get's reserved for this transaction in other words the work process is private to the transaction.

The work process will leave the Priv Mode and will be usable by other transactions when the transaction allocating the memory is closed.

abap/heap_area_* will limit the amount of memory allocatable on heap. Setting them to very low values might help you to control overall resource consumption, but the transactions will fail when they need more memory.