Delta (or triangular) shaped screw-links are great for rigging a wire strop round an anchor - one eye of the wire comes in at each of the top corners and another connector can be attached in the bottom corner
This shape is also good for connecting webbing to a fixed attachment point - can be used to connect their fall-arrest lanyards to the D-rings on harnesses
Doesn't readily unscrew - can be secured with a spanner or thread adhesive