The Droid Prisoner Compartment was a module contained at the back of each Reconnaissance Troop Transporter used by the Galactic Empire. It was used for the detention and taming of rebellious droids.
The Imperial troop transporter comic book that came packaged with Kenner's 1979 Imperial Troop Transporter toy also refers to this compartment in one instance as a Prisoner Immobilization Unit, which is actually a different device identified in the toy's instruction manual and on its box. The correct name is used later in the story.
The manual and packaging of the slightly modified and renamed "Imperial Cruiser" toy from Kenner's The Empire Strikes Back toy line only refer to this as a "compartment" and show it being used for a human action figure.