They are a good way to keep your filter holder slim and avoid Vignetting and stray light- particularly when using the Polariser at the front of your holder.
They are made using the same hands-on process as the Lee Filters ND grads with optically correct resin.
Combination filters typically feature a warm-up or colour correction filter one end and an ND grad the other end.
These have separate colours top and bottom and can only be made with hard grad transitions.
For example a 0.6ND to 81B combination filter both warms up the cool tone of a subject and controls contrast of light in the photo.
These have the warm-up over the entire filter, then the ND is overdyed at one end. The result gives a warming effect to both the sky and foreground with the ND balancing the exposure. Can be made in hard or soft transition according to your requirements.