The equator curve is only visible in the spruing menu is active to add, move, or remove sprues from a unit. This menu is activated when you left click on a unit.
The equator curve is design to help guide you place the sprue in the optimal location to minimize any undercuts. In the clip below you can see the undercut area highlight update real-time while moving the unit to show where residual material may be left behind. The purple undercut highlight stops as the equator curve because it is possible for the machine to mill from the other side.
Equator curve is not continuous or looks broken
Sometimes equator curves are not connected continuously around the unit either due to the complexity of the shape or an undercut area is disconnecting it. It is designed to be a tool to help guide you to place the sprue in the optimal location to minimize creating an undercut area with your sprue location. You can ignore it or rotate the unit to remove the undercut.
The clip below shows how the highlighted purple area that indicates an undercut area is located in the path of the equator on the contact after making an adjustment. For this example, it is just to demonstrate that the path can be made continuous but is not necessary to fix.