Status 54 and 55 in IDoc mean?

Hi ,

Could you please tel me as to what the status 54 and 55 mean ?

Well I know

54 : Error during formal application check

55 : Formal application check OK

But my question is what does it mean ?

Is it the transfer between ALE and application layer ?

How does it happen and when it happens ?

And at which point of time the status of 54 and 55 get populated.

Thanks

Kannan