Quick answer
Back-to-back (DB) and face-to-face (DF) pairs orient angular contact bearings in opposite directions to support axial loads in both directions. A tandem (DT) pair places the contact lines in the same direction to share axial load in one direction; an opposing support may still be required.
Back-to-back: DB
The contact lines diverge away from the pair. Bearing spacing, mounting fits and preload influence moment stiffness. Specify a matched set where the design requires one.
Face-to-face: DF
The contact lines converge toward the pair. Selection must account for alignment, support geometry and the manufacturer's arrangement requirements.
Tandem: DT
The bearings share load in one direction. Matching, axial support and preload cannot be inferred from two individual base model numbers. Include the arrangement and full set designation in your RFQ.