In that sentence, it refers to major in "a major online retailer" → "a major river". From Google's definition (no idea where they pull it from): informally, use

In that sentence, it refers to major in "a major online retailer" → "a major river". From Google's definition (no idea where they pull it from): informally, used to indicate that something already said is applicable a second time. ditto can be used as a noun (meaning the same again, e.g. in a list when you don't want to repeat something in the row below) or an adverb (as it is here, meaning likewise).