The Reset Material Assignments option can be used as a shortcut, to reset the material assignments for ALL material zones of your model, to the default (i.e. all material zones will have the properties of the first material in the Define Material Properties dialog).
You may find this useful if you have a complex model with many different materials, and you want to quickly reset all material assignments, so that you can start again from a known state.
If you accidentally select this option, remember that you can always use the Undo option in the Edit menu, to Undo any actions in the Slide Model program.
NOTE: the Reset Material Assignments option only resets property assignments (i.e. assignments made through the use of the Assign Properties option). The actual properties which you have defined with the Define Materials option are NOT affected and will NOT be reset.