The categories of the understanding are used “immanently” to refer to possible given experiences. Categories are used in principles of the understanding, which Kant tried to validate in his “Analytic of Principles” (beginning at A148/B147). The use of principles of understanding “is wholly immanent, because they have as their subject only the possibility of experience” (A308/B365).
In practice, the limitation of the use of the categories and associated principles lies in their application only to finite series of objects in space and time. Any member of the series can in principle be, so to speak, “reached” from one’s present situation. However, what cannot be reached is the totality of the series. For example, the application of the causal principle (all changes in the world are subject to determining rules) to events in the world is immanent, but the attempt to apply it to the totality of the world would not be an immanent use of the principle.
The immanent use of the understanding is opposed to its transcendent use, which extends beyond possible experience.