If shares of common stock are issued at a market price greater than par value, the amount in excess of par is credited to "b. Additional Paid-in Capital."
This represents the amount of capital that investors have contributed above and beyond the par value of the stock. The par value is essentially the nominal or face value of the stock, and any amount received above this value is considered additional paid-in capital.
Par value is the nominal or face value of a share of stock. It's an arbitrary amount set by the company and is typically quite low. For example, a company might set a par value of $0.01 or $1.00 per share.
Market Price is the actual price at which the company's shares are sold in the open market. The market price is determined by the forces of supply and demand.