The supply curve can be either for an individual seller or for the market as a whole, adding up the quantity supplied by all sellers. The quantity supplied is for a particular time period (e.g., the tons of steel a firm would supply in a year), but the units and time are often omitted in theoretical presentations.
Innumerable factors and circumstances could affect a seller's willingness or ability to produce and sell a good. Some of the more common factors are:
The Law of Diminishing Marginal Returns (LDMR) shapes the SRMC curve. The LDMR states that as production increases eventually a point (the point of diminishing marginal returns) will be reached after which additional units of output resulting from fixed increments of the labor input will be successively smaller. That is, beyond the point of diminishing marginal returns the marginal product of labor will continually decrease and hence a continually higher selling price would be necessary to induce the firm to produce more and more output.
The market supply curve can be translated into an equation. For a factor j for example the market supply function is
Note: not all assumptions that can be made for individual supply functions translate over to market supply functions directly.