What is "Sign" in terms of Saussure and what are the fundamental assumptions for it?
In terms of Ferdinand de Saussure, he originated the two-part model of "sign". He posited that each sign in linguistics has a twin aspect. Saussuere's definition of a sign consisted of the form the sign takes - its signifier - as well as the concept that the sign represents - the signified. Consequently, the sign which we identify with is the association of these two aspects.
The essence of a sign is the understanding or comprehension of it. As an example, a sign form can be a written word or a spoken word or sound. For example, a person can write or say the word "SCULPT" to another individual or group of individuals. This is the signifier. Now, this means nothing to the intended audience unless there is "the signified", in other words, the concept one wishes to convey.
The concept here for "SCULPT" is to perform a creative action to construct something of artistic or commercial merit. This would not be a "Sign" to a newborn baby, or a person who does not understand the English language. They would only hear the "sound" of the sign; they would not comprehend the sign.