This allows the builder to check that the cranks and chainrings will clear without having to install a bottom bracket and cranks arm. It’s designed around being a little pessimistic so the cranks length is 177 and the rings are 48/32 it assumes that you are using the recommended 110mm bottom bracket. It should snap into a standard english threaded shell. The middle has a slot to clear the rod on some frame jigs. It’s made out of acrylic so easily filed if necessary.

EDIT: Added a T47 version. Also works for PF30