I have always thought that engrams were decided when they dropped, but I now have a new theory. My Titan is always lacking plasteel plating from where he is a newer character and he went 20-32 really fast. I received a bunch of rare engrams doing the NFx3 and Weeklyx3 today, so I vaulted all of them to see if my Titan decoding them would provide titan armor. (My other 2 toons are warlock for those who don't want to check.) I got them out of the vault and to my surprise they [b][u]ALL[/u][/b] turned into Titan armor. So my theory is this, engrams are pulled from the current pool of items on drop, but not decided until they are decrypted. So basically you can't hold onto them until a new expansion comes out and get new gear because their loot pool has already been decided, just not the specific item. Hopefully this helps anyone who has a character that is class-materials poor like me and doesn't want to spend glimmer buying it