In a cannabis indoor grow room application, the difference between "harvest at once" and "perpetual harvest" is as follows:

Harvest at once: plants are brought into a grow room space where they stay as they grow from young plants through the flowering process and then are harvested. Then the room is cleaned and prepped for a new batch of young plants to be brought into the room to grow until flowering stage.

Perpetual harvest: plants are moved in and out of one grow room space to another grow room as the plants grow and will not stay in the same grow room during their full period of all stages of growth.