Hi
First the key words in the link provided by DQDICK are “temporary service equipment” in the last line of the initial description. When used as a temporary service unit on a construction site it would not be uncommon to have several subcontractors using all three outlets at one time so the unit is capable of delivering 100 amps continuous.
This constitutes a 125 /250 volt 100 amp service.
Now I will try to explain what looks like a misconception on the part of some people.
This is an electrical drawing of a 200 amp transformer like the power companies use on the pole outside your home or in a cabinet on the ground in an underground service.
The 50 amp 120 / 240 volt “service” provided at RV parks have a single compound circuit breaker with the capability of delivering 50 amps on one of the 2 lines at 120 volts.
We will call this 50 amps on line 1. Line 1 is using the neutral or center line in the drawing for its return to the transformer. If you put an Amprobe test meter on line 1 and neutral it would read 50 amps on both wires.
Now we will add a 10 amp load on line 2. If you put a third Amprobe meter on line 2 it would read 10 amps but the meter on the neutral would still read 50 amps and the meter on line 1 would read 60 amps. This is the result of the neutral being common to both line 1 and line 2 so the added load on line 2 appears to the electrical circuit as a 240 volt load and uses line 1 for its return to the transformer.
If the circuit is protected by a 50 amp compound breaker then the breaker would trip because the load on one side had exceeded the breaker capacity.
Now to further illustrate this lets put a 25 amp load on line 1 and a 25 amp load on line 2. The meters would read 50 amps on line 1 and 50 amps on line 2 and 0 amps on natural.
The result is a 50 amp 120 /240 volt service will only delivery 50 amps if the circuit is protected by a 50 amp circuit breaker.
I am sure this is as clear as mud but I gave it a good try.
Phil P