Iltizām (الالتزام) signifies a meaning external to the term's stipulated sense and its internal components, yet connected to it by an intelligible necessity. For instance, a conception such as a roof may lead the mind toward the support on which it rests; that support is not a constituent named by the word itself.
A spontaneous association is not enough. “Book” might remind one of a market, a pen or a school; such recollections do not thereby become necessary implications. The relevant connection must be justified.
The textbook introduces this distinction as preparation for metaphorical and indirect expression, where the audience moves from the surface sense of an expression to another intended meaning.