The rectangular detector is a detector bank that is marked as being a regular rectangular bank. It is a particular class in Mantid (RectangularDetector).
In order to be a RectangularDetector, a bank has to have these characteristics:
All pixels are the same size and shape (can be any size/shape)
The pixels are regularly spaced (the X/Y spacing can be different)
The pixels form a rectangular array (there can be gaps between pixels).
Several instruments including TOPAZ, PG3 and SNAP instruments use RectangularDetectors.
See the InstrumentDefinitionFile page for instructions on defining a rectangular detector.