Private factories have a limited number of workers. Workers can be paid in a firm wage with money or by specifying obtained part in percents. For example your wage can be 75%, that means you, as a worker will get 75% of what you obtained (after taxes to the budget) and factory owner will get 25%.
Private factory level
Factory owner can upgrade its level. Every factory have experience parameter, which increases every time someone works on the factory. As this factory experience grows factory owner can spend gold and upgrade his factory level.
Factory level has influence on productivity, factory storage size and maximum number of workers.