PCell callbacks and cdfgData variable

I want to execute the callback from a cell(pcell) after instantiation it to a layout, in order to check if chosen parameters are correct (no callback validation). 

evalstring(callbackOfPcell)

and it always returns t, even for the obviously mistaken setup for the inst. parameters. 

I've checked the source code of the "callbackOfPcell" callback function, and for getting the value from the cell it's using the cdfgData variable. The documentation and internet didn't give me an answer how cdfgData knows which cell is under test now.

So, the question is how cdfgData gets the right value for a cell, and why it has nil value after executing it in the CIW.