Placards, decals & markers
 

Reading chapter 20 of a 737NG maintenance manual, there's a procedure for removal & installation of "pressure sensitive decals", "pressure sensitive placards" & "aluminum foil markers".

What's the difference between placards, decals & markers?

Materials they are made of, location on the airframe, application and removal techniques and what their purpose is. I know what you mean, it isn't clear unless you refer to ATA 11. Decals tend to be external, placards are cabin/internal. Most are mandatory markings and can also be operator specific (Eglish and operator native language for example)l markers are also mandatory (tend to be system indicating such as flow direction of components etc)