Reading the ERS about vangs, there appears to be a contradiction between the intent of F 1.6 and the reference to a GNAV in F1.7.

F 1.6 says rigging works in tension only. F1.7(b) (xi) added GNAV to the description of a vang.

GNAV is vang backwards - a vang is below the boom and pulls downward - ie in tension, while a gnav is mounted above the boom and pushes downward - ie in compression. In both cases, they are used to control the angle of the boom.

Here below are the extracts from the ERS.

John

F.1.6 Rigging

Any equipment attached and/or connected at one or both ends to spars, sails or other rigging and capable of working in tension only. Includes associated fittings which are not permanently fixed to a hull, spar or spreader.

 F.1.7 Rigging Types

(b) RUNNING RIGGING

(xi) VANG  Equipment connected to the boom and used to control the angle between the mast and the boom. The term includes Gnav variations.