Adjustable wave solder fixtures are generally used in :-

small volume high product mix shops where a large variety of boards are being run
(and you want to keep a constant line width)

or

where the type of wave solder machine requires the use of some form of carrier being used
(Blundell and Seho).

Because of the need to work with a variety of board thicknesses the preferred fixing for this
type of carrier is the Titan Clip. The spring incorporated into this design also allows for some
board expansion - (don't force the board into the carrier - a light "click" is about right).

These can also be used as Mother Carriers with smaller pcb's or carriers fitted into the
unit.