What do 'FDCU' and 'FDCB' stand for?

‘FDCU*’ refers to a single set of beginning and end of range switches. These are usually connected to a relay or a (fire) management system. They reflect the position of the damper blade.
‘FDCB**’ refers to a double set of beginning and end of range switches. This solution is useful for example to directly control the fan and to communicate the damper blade position to a (fire) management system.

* FDCU is short for “Fin et Début de Course Unipolaire”
** FDCB is short for “Fin et Début de Course Bipolaire”