- " "Would you like us to take you inside?" Reese asked. / Foley was surprised at that. She guessed, once the citizen [Jennifer] was awake, regulations would allow you to offer assistance. Or maybe her partner still had some feelings after all. / "Oohhh —" The woman's eyes almost crossed. Foley guessed she would have trouble standing, much less walking. "Okay," the citizen added weakly. / So now they could use the identiplate. Sometimes, Foley swore, she could make no sense at all out of those numping regulations! Foley gently picked up the citizen's limp hand and pressed the woman's thumb into the plate below the doorbell. The door shooshed open. / Reese and Foley each took one of the woman's arms and helped her inside. "
- —From Back to the Future Part II by Craig Shaw Gardner (quote, page 63)
Thumb pads were a primary form of entry into a building or room in 2015. Rather than a doorknob which had to be turned and locked, a thumb pad identified the user by their unique thumbprint and welcomed the person once they passed security.
Because Jennifer Parker's thumbprint never changed over the years, Officers Reese and Foley of the Hill Valley Police — having verified her identity with an identa-pad — assumed she was the Jennifer of the future and she was able to enter her home.
In 2015, cash was rarely used as, by this time, thumbprints had also become an alternate form of currency; these were used like credit cards for transactions. Portable thumb units carried by charity collectors — one of whom was Terry — enabled people to give donations to such worthy causes such as the Hill Valley Preservation Society.
However, thumb bandits were known to steal people's thumbs to use them to gain access to secret or private areas. This was no doubt the reason why one of the messages displayed on the electronic readout on the hatbands of Officers Reese and Foley's peaked caps was Do not covet thy neighbor's thumb!
Behind the scenes
- In the Back to the Future Part II novelization, the thumb pad is referred to as an identiplate (see Quote above).
- Door locks with fingerprint readers are available from a number of manufacturers, including Samsung. Locks typically include touchscreen keypads as an alternative access mechanism. 
- Apple Pay and Android Pay are smartphone apps enabling electronic payments, secured by the user's fingerprint.