I am starting my technical training with TDI and it requires a 50lb lift bag. I think the purpose of the bag is redundant buoyancy, as well as for decompression stop line, but I'm not 100% as I haven't actually taken the class yet. Are there any features I should be looking for that make one better than another for the purposes of technical diving (versus search and recovery of objects)? Do you have any recommendations for lift bags? Thanks.