The Plate Selector dialog box displays all the colors that can be separated out of the document and printed as separations. It lists:
Spot colors specified in the Color Plate Specifications dialog box.
Colors to which a picture has been assigned.

• Spot colors embedded in EPSF files, if you've selected the separation option 'Use EPSF Info' in an image's Sep Options dialog box.

• The CMYK colors, if at least one color in the document is marked as 'process' or if there is a picture block in the document.

As standard, all plates are set to print. RSG automatically prints only those separations which contain separation imaging, thus economizing on printing materials.
You can specifically deselect a color plate to stop it printing by clicking its color bar in the Color Plate Selector dialog box, so that its checkmark disappears. All Plates lets you set all plates to print if not all are selected.