AGPL or Actual Ground Position Line as the name suggests is a line demarcating Indian and Pakistani troop position.The troops, who moved into the areas after 1984.Hence their no question AGPL being demarcated in previous treaties.
The Siachen region was not demarcated in 1972 agreement, as it was then considered insignificant, an areas incapable human habitation,back then it served very little strategic purpose to either army.
I have been involved in this mess for quite a few years so I know the details around the entire AGPL story. Simplistically, the AGPL formula is being used to extend a status similar to the LoC across the glacier. As I have mentioned before, the LoC and the AGPL are all open to interpretation and as such violable (both sides have violated the LoC and even the positions on the glacier were being adjusted up until 2003) as the sanctity of Shimla agreement has been breached hundreds of times by now. Lets also be clear that Shimla agreement does not equate the LoC to IB in any way regardless of this being wishful thinking on the Indian side.