The plumbing sub is responsible for all the pool equipment and PVC plumbing necessary to take care of things like pool filtration, pool cleaning, chlorination systems, waterfalls, pool/spa heating, spa jets/air, skimmers, aerators, auto water-levelers, drains/returns. This would include all the pumps, filters, gas heaters, and chlorinators.

Typical Spa / Pool combination plumbing layout:

You want to make sure all piping is a minimum of schedule 40 PVC or equivalent and stamped with ASTM approval. The plumber must determine the sizes of all piping by calculating the volume of water in the pool. Water is to be filtered every six hours of operation. Over-sized piping is a key to an efficient and easy to maintain pool. A good rule when in doubt, use 2" minimum. This may cost you more, but you will reap benefits in reduced pump size and pump run times.

Most pools will have one or more skimmer boxes for drawing the water from the surface of the pool. (normally allow one skimmer per 50SqM of pool surface area.

The skimmer box should be mounted so that the mouth faces the prevailing winds, the skimmer box does not have to be in the deep end of the pool but must work with the wind which is blowing leaves etc across the water surface. The skimmer box should be mounted such that the center of its mouth is 150mm below the top of the pool. Make sure that the skimmer box is set level & upright. Connect a pipe to the suction point on the skimmer box, this pipe will have to go back to the area where you have your pump & filter. (normally 50mm PVC pressure pipe)
Connect a pipe to the drain connection on the skimmer box, this pipe goes to the main drain in the base of the pool. (this pipe is normally 40mm PVC pressure pipe.) Make sure to seal the front & top of the skimmer box to prevent concrete or concrete slurry entering during concreting.
(Some pools have an overflow gutter or weir or level deck, these don't have a skimmer box, see here for information)

Make sure to seal the top of the main drain to stop concrete or concrete slurry getting in during concreting. The main drain must be protected from entry of cementous compounds for the entire construction period.

The main drain is a unit which is set at the lowest point of the pool. The main drain has a perforated pipe in its base which is set in the gravel in the pit at the deepest point of the pool. There is also a pipe which connects from the side of the main drain to the skimmer box. Set the main drain level with the deepest point required in the pool.

Some other considerations to be made are:

Pipe Type - It is normal to use UPVC pressure pipe not Sewer or Waste pipe for swimming pool applications. The use of waste pipe can cause premature failure both above & below ground level. It is also preferable to use Class 12 pipe not Class 9 as it has greater mechanical strength in case you hit in in the future as you are digging the garden.

Pipe Size - The size or diameter of the pipe is related to how much water you want to pump through it & how far you want to pump the water.
As a rule of thumb, you can say if the pump is smaller than 600 Watts (3/4 HP) then use 40mm (1.5") PVC Pressure pipe & fittings, if the pump is bigger than 600 Watts (3/4 HP) then use 50mm (2") PVC pressure pipe. The difference in cost of pipe & fittings is very little but the possible performance difference from your pump can be massive.

Horizontal Position- You will have to have the pump at least 3.0M from the waters edge unless there is a solid wall between (see AS3000)or your local wiring regulations. You may take the pump as far as you like but if you go over 10M then you will have to increase your pipe size especially on the suction side.

Vertical Position- You can have the pump any where between 2.0M below the water surface to about 500mm above the water surface with out any real problems. If you are below the surface, then you will need to install a valve in the suction line in front of the pump & a check valve in the line leaving the filter & returning to the pool.
If the pump has to be more than 500mm or 20" above the water level, then you will need to install a check (non return valve) in the suction line as close to the skimmer as possible.
This check valve must have flanges or unions each side of it to allow it to be disconnected for cleaning.

Pipe Installation - All pipe work except for the last few hundred mm or 8" should be level & below water level. If you run your suction line above water level on the horizontal then you will have problems with priming the pump & the pumps seal could burn.

Joints - All joints must be glued correctly in accordance with the pipe & fitting manufacturers recommendations. Always make sure that the joints are square & all the way in.

Testing - There are people in most states of the USA & Australia now who can come to your property & test your pipe work before it is concreted in or covered over, this costs a small amount initially but can save thousands later.

Backwash Lines:- The length & size of these are critical, never make a line any longer than absolutely necessary. If you are using a 600Watt (3/4 HP) pump the maximum is 6.0M or 20' on the horizontal in 40mm or 1.5" pipe, after that go to 50mm or 2" pipe. If using a pump larger than 600 Watts (3/4 HP), the use 50mm or 2" pipe all the way for up to 12 M or 40' horizontally, if you need to go further than that then use 80mm SWV PVC or 3" sewer or storm water pipe & fittings.