It can be used in synchronous clocked systems but it is an asynchronous T flip-flop. If we consider it as a D flip-flop with D input connected to some output (in this specific case, to its own inverted output) and C input connected to the clock, it is a synchronous D flip-flop having two inputs (data and clock). But in this circuit the D flip-flop is only a component used to build a T flip-flop having only one input (toggle). So, this circuit is an asynchronous T flip-flop that can be used wherever.  , , email)