Please read 1.9 and 1.17 for a better understanding of this sutra. In 1.9 the meaning and definition of “vikalpa” are given How we tend to form an imaginary world because of words and knowledge.
In 1.17 it is mentioned that vitarka is the state of application of mind towards an object. It happens in the initial stages of meditation.
Here in this sutra, two mental processes are explained. Imagination and application of mind on an object. The object here is knowledge which is in the form of words and has a meaning.
Thus we can say that sampatti ( Samadhi)/ absorption state, is achieved by meditation when the mind is able to concentrate on an object which has a meaning and can be expressed through words.