Both of them is a concept that can be used as adjective undefined or indefinite pronoun, always in plural. The term is used when, Considering a couple of elements, both are referred to: namely, to one and the other.
Suppose a young man walks into a store of clothing and it is tested two pants, one black and other blue. As he leaves the dressing room, the salesperson asks him which one he will take and the boy replies “both of them”. In this way it indicates that will buy the black pants and the blue pants.
Take the case of a novelist who, thus far, presented two books. Said plays were published through the Editorial Mundilitario. Therefore, it can be stated that both books were edited by Worldly.
As you can see, the idea of both refers to two options or items when that’s the total number existing or available. If there are three alternatives and someone chooses two, you cannot talk about “Both alternatives”, because it is impossible to know which ones are being pointed out.
The notion of both can also appear as the plural of the noun both. In several South American countries, an ambo is a costume composed of a trousers and one jacket used by men. For example: “In the closet I have four or five both, you can choose the one you like the most and I’ll lend it to you”, “I don’t like both that they offered me in this place, I’m going to keep looking”, “The players wore their both shiny at the party organized by the president of the club to celebrate the obtaining of the championship”.