If B requires A, the B is dependent on A. Be is a dependent.

Dependent: (adj) determined or conditioned by another : contingent (e.g., plans that are dependent on the weather) Merriam-Webster

The corresponding noun has the same spelling.

If B requires A, then B is dependent on A. So B is a dependent of A.

Dependent: (adj) determined or conditioned by another : contingent (e.g., plans that are dependent on the weather) Merriam-Webster

The corresponding noun has the same spelling.