1) As per Stop list title, adding words here will exclude them from the second product title (which is made up of the original one):
You can add new stop words that are often included in the titles of products belonging to your niche, but do not suit your store and your target audience. For example, your products shoud be all in English, but in titles you often see keywords in other languages - nuevo, chico, chica, etc. In this case, it's reasonable to add them to your Stop list.
If you prefer to use the first product title (composed of image-specific keywords) or edit the title manually, it is not necessary for you to add anything else in the Stop list title.
2) As per Item specifics, you can add here the most popular specifications for products in your niche (if they are not already added by default): Applicable gender, Pattern Type, Fit, etc. (Take a look at specifications of your products and select those you want to mention in your descriptions.)
You can also consult this article to learn more about Sellika settings in general.