Transfer stack in slot 1.7.10

[Kander16]

Hi,

 

Could somebody please explain what the Transferstackinslot method does?

Because I cannot put fuel in my fuel slot when shift clicking.

I also can't place items in the output slot with dragging, but I can when I shift click.

 

TransferStackInSlot:

 

http://pastebin.com/MPSVUcNx

 

Especially this code I do not understand well:

 

if (!this.mergeItemStack(itemstack1, 3, 39, true))

 

Thanks.