"P" as in "P90" etc actually stands for Probability, ie there is...

"P" as in "P90" etc actually stands for Probability, ie there is a 90% Probability that actual producible oil / gas will exceed the P90 number. So P50 is the median (for any statisticians out there), and there is only a 10% estimated chance of reserves exceeding a P10 number.

Just to really confuse things though, you'll often see an alternative terminology:

"1P" = "Proven" - aligns (I think) with P90
"2P" = "Proven and Probable" - aligns with P50
"3P" = "Proven, Probable, and Possible" - aligns with P10

I'm not sure that the definitions are exactly the same, eg 3P may not mean exactly the same as P10, but near enough for most purposes I think.