PLKO -tasklist header-Internal counter???/?

Hi,

Can any one could explain me what exactly is this field

"Internal counter"?

Also it would be great if any one explain me the linkage between the tables PLKO PLAS & PLPO????/

I know the key fields by which these tables can be attached !

like Task activity type,Group counter number and so on !

But i want is exact link by which these tables c an be linked

Raj