OK CODE – In SAP, an OK code is used for any functionality. The OK code is used to perform an action executed or will be executed (save, back, exit) etc.
BU – This OK function is used to perform SAVE function in an MM01 transaction.
ZK – This code is used to get the additional information.
To find out a function of each button, you have to navigate to System -> Status -> Double click on GUI Status.