Whether to include the build closure of the whole system in its runtime closure. This can be useful for making changes fully offline, as it includes all sources, patches, and intermediate outputs required to build all the derivations that the system depends on.
Note that this includes all the derivations, down from the included applications to their sources, the compilers used to build them, and even the bootstrap compiler used to compile the compilers. This increases the size of the system and the time needed to download its dependencies drastically: a minimal configuration with no extra services enabled grows from ~670MiB in size to 13.5GiB, and takes proportionally longer to download.