speed mode is a vertical mode of operation, so first you set the altitude you want to get to and then engage speed mode - FD will command AP to get to selected altitude at your given speed (either by climbing or descending, you'll see CLB or DES in PFD annunciator area). with this mode of operation you'll notice your VS being the variable set by FD to achieve your target speed. on the other hand if you climb or descend in VS mode you set your target VS and your speed is the variable that will arrive at some value (not controlled directly by you) to achieve proper VS. so basically two ways of achieving the same thing, you just control different parameter leaving the other in the hands of FD and physics
i hope i made it clear enough, just yesterday was trying to understand speed mode, because it's not really what you might expect it to be. important thing to remember is that it is vertical mode of operation, not horizontal (so setting it up to 300kts in the cruise phase won't make the aircraft travel with this speed; that you may achieve only with your throttle operation, which in CRJ is left in pilot's hands with no automatic control).
speed mode will disengage itself automatically after reaching selected altitude (similarly to VS mode), after which FD will switch to ALT HOLD mode, which basically means that if you forget to lower the throttle after climb, the aircraft will level of and your speed will go through the red roof