You can have a union connection on one side or both of the pressure reducer. If only on one side, the other would be directly threaded or soldered (sweated) to the connectiong pipe or fitting.
The below picture shows an example of a union.

Here is a link that might be useful: Plumbing Fittings

Randy427

Thank you for your help. So a single union fitting would be a valve that has what you show on this picture on one side (INLET) which could be threaded or sweated and no union on the other side(OUTLET) just the valve threads. I hope I am right.

Thanks

Mario

My first question would be why are you changing the valve?

The reason that I asked the previous question is that far too often homeowners change out the entire valve when all they really needed to do is clean the internal sediment screens.

Now I am sure that many will say they don't have screens but that is not true,,,,Code requires a sediment screen on the input side of a PRV.

On mine, the union fitting is on the OUTLET side. I don't know if they come the other way.