M3 is the switch for a classical boost converter. To operate as a boost converter M1 must be closed and M2 open. Assume SB is left open. In a simple boost converter SA would be a diode, cathode to load.
The whole circuit, depending on the sequence of switch closures, can be either a buck converter or a boost converter. I would suppose the notion is that it could produce both a boosted output and a bucked output by alternating complete cycles.
