The catalog name for the Apple II floppy disk controller card used in every Apple II model (save the Apple IIgs).
There are 3 known versions of the card in circulation. The original model, used from the introduction of the 'II until 1983, is the famous piece of engineering by Woz where he reduced the chip count from over 20 down to 8. The second, used from 1983 until 1994, uses the legendary IWM controller chip to replace most of the chips, further reducing the count from 8 to 3. A third and seldom card used near the end of the Apple //e's life, circa 1992 until 1994, combined the remaining 3 chips into a single IC.
Regardless of model, the Disk II controller has connectors 2 20-pin connectors for connecting up to two Apple Drive II Disk drives. These connectors also supplied electrical power so no separate power cables were necessary. The card can be installed in any slot, but is traditionally found in slot 6 (hence PR#6).