Gas Rate eco tec sustain

[JTplumbingservices]

looking at the manual for an Eco Tec Sustain and the gas rate is in two sections, one for H gas and one for P gas.
Can someone explain what H and P is ?
I feel like this is something obvious I know but I have a mind blank

 

[ShaunCorbs]

Heating

Portable water

??

 

[Last Plumber]

looking at the manual for an Eco Tec Sustain and the gas rate is in two sections, one for H gas and one for P gas.
Can someone explain what H and P is ?
I feel like this is something obvious I know but I have a mind blank

H=Nat Gas
P=Propane.