It's for each gun, you have to prestige them individually to unlock this feature on them - when you do prestige you're weapons, camo and related challenges stay but you lose the attatchments, you'll have to unlock them again, same for each prestige
You need to prestige each individual weapon to put both the emblem and the clan tag on in.
For example. If I prestige my pdw once. I can now put my clan tag on my pdw. Prestige it a seconds time now I can put both my clan tag AND my emblem. But only on my pdw.
After you do that move on to your next preferred weapon and repeat!
Pro tip: if you get a weapon prestiged twice you get an awesome new calling card! And if you do all of one weapon type (like all smgs) you get diamond camo!!!
Haha have a good one.