If I have a bunch of ultrahigh-frequency (UHF) and high-frequency (HF) RFID tags stacked one on top of the other, can I still read each of them?
Probably not. The antenna from one tag would likely block energy from reaching the antennas of the tags behind it, and the tags lower down in the pile would not be able to respond. If, however, you had a stack of tags (on paper files, for example) and each was in a different location within the stack, then energy could reach all of the tags and they would be able to respond.
—Mark Roberti, Founder and Editor, RFID Journal