The A pins can connect to four independent lines that expect or send a 3.3V logic signal. The B pins will connect to four lines that expect or send a 5V signal. You use only the pins you need to interface a 3.3V object to a 5V object. For example, B1 could connect to a processor's 5V SCL pin while the corresponding A1 could connect to an object expecting the same signal, SCL, but at a 3.3V level. You can use any of the four Ax/Bx pairs for any signal. The level conversion is electronic and entirely within the circuitry on the board. Your sketch does not have to do anything different. The board is the easiest way for a 5V object to talk to a 3.3V object in a bi-directional fashion. The software doesn't know it's happening.