This is a copper jumper strap normally used with the ground wires in the spa. These are mounted on the back of the pumps connecting the ground wire. Is part of the connections from the heater to the PC Board.
The pressure switch in your spa controls the water pressure in your spa. The pressure switch harness is a replacement part for the original pressure switch harness that allows the pressure switch to be connected to the controls of your spa.
The Drain Plug for Power WOW is a wet end drain plug for the Pentair Power WOW. This part does not come with an O-ring. The part allows the pump to be drained of stagnant water so protect it from damage and to keep it running for as long as possible.