Strange (flyback?) diode in a circuit with a transistor switching an inductor
The circuit is "strange" and, as far as I can see, far from optimum.

The diode does not make sense where it is located and serves no purpose. When the TIP42C (datasheet here) is turned on the inductor has ~= 12V applied. When the TIP42C turns off the diode side of the inductor will "ring" positively to a voltage above the 12V supply. The diode will be reverse biased as shown and does not affect the ringing/ flyback voltage. It does nothing and may as well not be there.

C1 is a filter or reservoir capacitor.  It especially acts to reove noise from the supply when the motor is running and provides current peaks to the motor. (Both the previous actions are "different sides of the same coin"). 

The transistor is connected in an extremely unusual manner. The inductor is in its emitter - it is an "emitter follower. There is no obvius reason for doing this. The transistor is turned on by clamping its base to collector. This means that the emitter must be above ground by a Vbe drop. As the emitter approaches the base voltage the transistor starts toturn off (at about 0.6V to 1V above ground range - so the transistor "wastes" some of the supplu voltage.

A better arrangement would be to have the emitter connected to V+ and the motor/coil in the collector circuit. 

Overall I conclude that either :


The circuit is drawn incorrectly (happens :-) ) 


or


The person who "designed" it has very little understanding of electronics. 




Can you provide a link to an A3144 data sheet please? It is not fully clear how it works.