Signals approach control/limited aspect

[amtrack505]

Hello,
In railworks if you double click a signal a tab out box will appear containing a couple tick boxes and typing boxes, i've been messing around with the tick boxes(see pic), seeing what they do. They state approach control [ ] limited aspect [ ] in that order. Messing around with the boxes i've found some new signal combinations Ex. yellow over yellow. So does anyone know what this does? Im am trying to make a advanced approach (flashing yellow) with SMM's searchlight signals i just cant seem to do it any help or information about this tick boxes?