perhaps a few pictures will make it clearer...
its easy for people to get mixed up between the different named parts, more so if they havn't seen them before; which are basically four different things shown here, in these pics; plate holder, film or often called sheath holder, film pack holder and lastely the film pack itself
pic 1 shows a plate holder with a glass plate negative inserted, the darkslide for the plate holder is to the right.
pic 2 shows the glass plate being removed, acheived by inserting your finger at the top and downward pressure against springs in the botton of the plate holder. doing that frees it from the top and can be lifted out easily
pic 3 shows a film sheath/holder being inserted in the plate holder.
pic 4 shows it fits in by the same manner in which you would insert the glass plate negative. by inserting the bottom first against the springs in the plate holder
pic 5 shows, which is now in effect become sheet film holder (and the darkslide being inserted. unless you want to use glass sheets any time in the future the sheaths never need to be removed again. the sheet film slides in under the edges of the film sheath.
pic 6 shows the film pack holder (bottom) with its darkslide inserted (mentioned as a clam shell earlier on). and (top) a film pack that has not been used (all the tabs that can be seen)
pic 7 shows the film pack inserted in the film pack holder ready to be placed in the camera ready for use.
the film pack holders have little to no use nowadays. the film packs are no longer available. even though on ebay people list pack holders for sale! they may as well give them away