The mobile options page currently uses the string "Please enter a URL link" as a message displayed if no filter list URL was entered. According to current spec, this message should say "A URL link is required" which isn't exactly better. "URL link" makes no sense, it's two terms meaning the same think. So it should be either "A link is required" or "A URL is required" (probably the latter given that the page already uses the term "URL").
Note that the same bogus term is used in the sentence "The fields should auto-populate with the filter subscription title and URL link" of the spec, should probably be corrected there as well.