A wet plate holder would hold a glass plate or tin plate. A film holder holds film. I own an 8x10 but I shoot film. I've never done or been around someone shooting wet plate photography. Watch this video. If you are interested in doing wet plate then there are a lot of videos on YouTube. You could also join the Large Format Photography Forum where some members shoot wet plate and would be happy to help you. There may be wet plate shooters here as well.
Wet plates can be any size, postage stamp to huge measuring 14x17", 16x20" and larger as well as many intermediate and non standard sizes. An 8x10" filmholder is just that, a (usually double) holder for 8x10" sheet film.
you can make a wet plate holder out of a regular old film holder
it is a simple modification but it isn't reversible ...
it involves removing the septum ...
here's a quick video