For both Raspberry Pi and Beaglebone Black, we use Primary to refer to displays that are able to display the desktop environment of the OS running in these Linux machines. These are essentially Monitors for these SBC's (Single Board Computers) and show the desktop or terminal or direct to a custom application etc as would a typical computer monitor. Primary displays typically interface to the host using SPI or raw RGB signals.
On the other hand, Secondary display solutions are not capable of the same feat in terms of displaying the Desktop, but are programmed using our Workshop4 IDE to design attractive GUI's which are then interfaced to with the Host Raspberry Pi or Beaglebone Black. The Host can send values to the display to update the GUI widgets, and receive information back such as touch events or slider/knob/button input, or the GUI could be completely drawn by the host if programmed as a Slave. Secondary displays typically interface to the Host using a serial UART.
Primary display examples are gen4-4DPI-xx, 4DPi-xx, gen4-4DCAPE-xx, 4DCAPE-xx
Secondary display examples are gen4-uLCD-xx, uLCD-xx, uOLED-xx, pixxiLCD-xx