The Rainbird 5000 PRS models contain a built-in Pressure Regulator which is marginally useful if you live in an area where excessive pressure is causing your sprinkler heads to produce a lot of mist, but for many customers this is not an important issue and use of the PRS Series rotor heads may create a problem by making it very difficult to flush dirt and debris through the head after a pipe break or during installation. Very few people seemed to want a pressure regulator in their 5000 or 5000 Plus heads, so we have made a decision not to stock or sell that product.

My own opinion is that pressure should be regulated if required, but at one point near the water source, not at each individual head unless extremely radical grades are being irrigated, and even then, I just don't think using a lot of pressure regulating heads makes much sense.

It's a lot more cost effective to purchase one pressure regulator for system-wide regulation, or several for individual watering zone regulation, than to constantly have to purchase heads containing pressure regulators in them.

The 5000 PLUS model is a slightly upgraded version of the standard 5000 model. It features a green cover as opposed to the black cover on the 5000 models and the 5000 Plus models have a built-in flow shut-off which allows you to shut off the flow of water from the head itself in order to allow changing nozzles without shutting off the entire watering zone, or you can use the flow shut-off to eliminate watering with that specific head.

The 5000 Plus model also is compatible with more options such as the Stainless Steel Riser. The Plus model is designated by the suffix PL following the model number as in 5004-PL-PC if it is a 5000 Plus Part Circle head, as opposed to the 5004-PC if it is standard 5000 Part Circle head. 

The 5000 Plus PRS model is the regular 5000 Plus model with the built-in previously discussed Pressure Regulator.

Bottom line, if you are a homeowner interested in replacing an existing head on a residential system, you will usually be correct if you stick with the basic 5000 or 5000 PLUS models. A Check Valve can be ordered from the pull-down option menu when ordering either the 5000 or 5000 Plus heads if that options is required.

The thing to remember with check valves in heads is they do stop low head draining during the summer months. But if you live in a hard freeze climate area, they can complicate blowing your system out and misting anti-freeze into it when preparing for winter freezes.