The rules of the understanding fall under the general scheme of (Aristotelian) logic.
Kant thinks of logic in the spirit of Aristotle, and his transcendental logic is similarly structured. [See B169]
The Analytic of Concepts
Why is the fourth principle of the categories (completeness) so important?
It is crucial for Transcendental Logic to have the full scope of its material in place in order to fulfill the certainty required of it.
Contrast with the necessarily limited (inductive) reasoning involved with (non-empty) empirical matters, and also with the necessarily empty (deductive) reasoning found in pure general logic.
Transcendental logic offers both deductive certainty and non-emptiness by leveraging the synthetic a priori nature of its objects.
Understanding – [discursive] capacity to know by means of concepts [by thinking][as opposed to intuiting]
Concepts can be special in that they can be used repeatedly.
Judgment is a kind of synthesizing of concepts (as unification). [More]
Note: Kant’s fur, willow, linden ‘under’ concept Tree [Example in his Logic – [Lectures on Logic, 640]]
For next reading: what does Kant have in mind when he writes of ‘synthesizing’?