Billetted soldiers were not included in the 1939 Register. The only active service personnel who were required to be enumerated were those who were at home on leave. Conscription did not begin until later, since one of the purposes of the register was to create an accurate list for this purpose. So the people already in the forces were regulars, volunteers, and reserves who had been called up.
I think there may be other categories which you have not envisaged.
Some people doing specialist jobs, such as Research and Development, may have been invited to be members of new teams being set up.
