Extension lines (sometimes 'projection lines' or 'witness lines') connect the feature being measured to the dimension line itself. They start with a small visible gap from the feature so the extension doesn't get confused with the feature's outline, then extend just past the dimension-line arrow.
Multiple extension lines often overlap when a stack of dimensions runs parallel. AS 1100 calls for them to be drawn in a thinner line weight than the object outline so they read as annotations rather than as part of the geometry.
CAD60 extension lines are placed automatically from the geometry. Start, end, and the gap from the feature are all computed.