If you want the slider to show only on a subset of your products, limited products is for you. Keep in mind that you can also use the checkbox "Show slider only on source products" so that we show your slider widget only on source products from your mappings. Navigate to "Products" tab, and then "Slider" on the left:
Click on the "Limited" button to choose limited products manually. The number inside the parentheses is showing how many products are selected.
Start typing the product name into the search box. Click the product below the search box to add it to the product list on the right:
Click "Save" to finalize your selection. Wait for the changes to be published to your site (the blue progress bar on top).