What does the fourth position mean here?

I'm doing a .tlc file and I have a situation like this in a random model:

Looping over: CompiledModel.DWorks, I'm getting a this_dwork for eah interation:

%foreach i = CompiledModel.DWorks.NumDWorks
    %assign this_dwork = CompiledModel.DWorks.DWork[i]

I'm in the DWork shown above (memory_2) for the first signal (I'll have one DWork for each signal arriving in the memory block), then I get this memory block with:

%assign grBlock = SLibGrBlock(this_dwork.GrSrc)

This DWork value (this_dwork) for curiosity:

{ GrSrc [0, 4]; LogStateName "PreviousInput"; LogicalSrc D2; MemoryMapIdx [2, 0, 2]; Name "PreviousInput"; SigSrc [0, -1, 1, 2]; TID 1; VarGroupIdx [2, 2]; flatElemIdx 0; hierInfoIdx -1 }

In this block (grBlock), I'll have two Regions, one for each signal:

> grBlock.DataInputPort.Region[0]
{ MemoryMapIdx [0, 0, 4]; _Source [B5] }
> grBlock.DataInputPort.Region[1]
{ MemoryMapIdx [0, 0, 5]; _Source [B6] }

From the this_dwork variable, how can I know which region (signal input) this dwork belongs?

I have this this_dwork.SigSrc[3], the fourth position of SigSrc, what this fourth position means?

(Is there a place where I can find a reference for these things?)