If you have no idea you likely have been using NON-CTL versions that have no rejection features and your tandems always fit in any slot you use.I use tandem breakers all the time, and I have no idea what the question is.
At one time yes. Eventually it got to where one cover sometimes worked on more than one loadcenter, and still is that way today. You can get a 20 circuit loadcenter that is using same cabinet and cover as a 24 circuit version, and even within those parameters one may be rated for tandems in every space, limited number of spaces, or not rated for any tandems at all.One way to tell if the panel will accept ctl breakers on sq d, is the panel sch on the door. If it has a dotted or thin line in the middle of the block, it will accept ctl breakers. And sometimes the panel number is an indication, ie 10-20 means 10 reg or 10 ctl. I agree there are many non ctl btrakers used where not allowed