Hi Alin,
Continuous Consumption (Fixed) gives you the number of capacities that will be consumed throughout the entire duration of the mode.
Example: you have a mode with variable duration of 765s and continuous consumption (fixed) = 1. If you create an order for quantity 3, the net processing time of the activity will be 2295 and throughout this period 1 capacity of the resource will be consumed.
On the other hand, if you were to have a fixed duration for the activity, it could make sense to have a variable continuous consumption e.g. the higher the order quantity, the more capacities are consumed within the same period of time. I haven't seen customers using this setup in APO.
Regards,
Tiago