Hi George,
Welcome to YamahaSynth. You should download and keep handy a copy of the P250 DataList booklet:
Link — P250 Data List pdf

This will give you a listing of all the XG Voices and their BankSelect/Program Change information.

XG is Yamaha’s extended set of General MIDI sounds and control options. Normal GM sounds come in 16 groups, each group has 8 Programs.
Bank Select MSB = 000, LSB = 000 and Program Change* = 001-128 is the info that defines the address of normal GM sound
Bank Select MSB = 127, LSB = 000 and Program Change — this is for the Drum Kits starting on Page 10

*The Program Changes are listed 1-128, the Program Number is one less — 000-127

For example, to recall the GM “Harmonica” which is listed as 23 you would use:
MSB = 000, LSB = 000, PGM# = 022 (subtract 1 from the Program listing).

Listed in the columns to right of the “Harmonica” are your alternate XG sounds… this way at the same PGM# but a different MSB / LSB will be the variation sounds. Not every parallel XG Bank contains an alternate (only where appropriate). At the very top of each column is a description of the alternate Bank.

“Harmonica 2” is found in the “Detune1” bank
MSB = 000, LSB = 032, PGM# = 022

Accordions are found
MSB = 000, LSB = 000, PGM# = 21
MSB = 000, LSB = 032, PGM# = 21 “Detune1” bank

NORMAL XG alternate sounds always share the same MSB (000) and Program Number as the GM list, while the LSB selects the XG Bank type. This way when using XG, if you used 000-032-022 which is the Detune1 Harmonica and some one else who doesn’t have an XG sound set, it will simply select the Harmonica in the 000-000 GM bank.