I'm not sure where that came from, but basically, in SFIV and SFV it was:

Light Punch: A
Mid Punch: S
Strong Punch: D
Light Kick: Z
Mid Kick: X
Strong Kick: C

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So for M.U.G.E.N i adapted this way:

A - Z
B - X
C - C
X - A
Y - S
Z - D
Start - V