Jinzen’s coupling sleeves are available with both half-bridge and full-bridge types, as well as adapter couplings.
Semi-bridge coupling, by far is the most popular, has a small non-threaded bridge in the center. The drill rod cannot thread past the center of the couplings, and the smaller diameter rods portions butt together in the center bridge area of the coupling. Semi-bridge couplings are most suited to high torque machines. Most rope (R) and Trapezoidal (T) threaded couplings are semi-bridged.
Full bridge coupling has a great advantage that it positively eliminates the potential for the coupling to creep along the threaded joints. These couplings, are typically used in a Trapezoidal thread, in surface drilling application, have better uncoupling characteristics and tend to maintain tighter joints. Full-bridge couplings have less chance of jamming and are best suited to machines equipped with independent rotation.
Adapter couplings are used when changing from one thread type, or size, to another and are typically required only in special circumstances.

Specification for Couple Sleeve