Spot UV is setup the same way as Foil and you must provide an additional file for each side with the spot UV or Foil made with 100% K only. Limitations: Spot UV on Silk Cards can only be maximum 40% coverage. Spot UV cannot be put on the back side of a print job that has Foil on the front. For Foil Guildlines, please view this link: https://405media.zendesk.com/hc/en-us/articles/204686917
Spot UV is always clear in color, yet you can place it on top of color elements in your artwork. The 100% K you specify is used to create the plate for the printing process of where the clear gloss UV coating will go. The gloss UV is named UV due to it being cured by UV light during printing not for blocking UV to your artwork.
In the following example below, a basic layout - A Front, Back, Front UV, and Back UV artboards in Illustrator (Similar layouts can be done in photoshop layers just as easy). In this example the 405Media.com logo would be glossy as well as the "affordable full color quality printing!" text. Also various keywords would also be printed only as spot UV as a design element - notice how those words do not show on the Front or Back sides. Thus anything you put on your spot UV artboards/layers (in 100%K only) will result in those elements being printed with gloss UV.
Please note, we do not recommend using spot uv on top of small print as spot uv tends to slightly shift and we do not guarantee alignment thus why our recommendation. Spot UV works best on top of photos to make them pop, and as design elements in itself to bring to life a design.