How to maintain Custom Fields using BAPI.

Dear Gurus,

I have searched in Google and I have got loads of websites which explains the usage of Extentionin.

But I did not quite comprehend it.

For instance, somewhere it was mentioned that

If you need to use custom fields for VBAK you must adjust the following structures for customer enhancements to table VBAK:  

a) VBAKKOZ  

b) VBAKKOZX  

c) BAPE_VBAK  

d) BAPE_VBAKX

If you need to use custom fields for VBEP you must adjust the following structures for customer enhancements to table VBEP: 

  a) VBEPKOZ 

  b) VBEPKOZX 

  c) BAPE_VBEP 

  d) BAPE_VBEPX

Fine, I need to adjust the following structures with Customer Fields, I see  point there, OK.. but how I on earth I would know what Structures I need to adjust when I am dealing with some other Bapi's and some other tables..

Is there a logic on which I have to adjust the respective Structures.. Please Advise.