If it needs the same amount of labour to run at half capacity then the production vector (–1,–5,4,0) would also be operationally possible.
The mill's owner will normally choose y from the production set to maximise this quantity.
If a production set is separable then we may define a "production value function" fp(x) in terms of a price vector p. If x is a monetary quantity, then fp(x) is the maximum monetary value of output obtainable in Y from inputs whose cost is x.
Constant returns to scale mean that if y is in the production set, then so too is λy for any positive λ.
There is no entirely satisfactory way to define increasing or decreasing returns to scale for general production sets.