What is a subsidiary signal?

Related questions

1. What do train whistles mean?
2. What is Recfm?
3. Do trains turn around?
4. What is Warner signal?
5. What is moving block system?
6. How do train signals work?
7. What is a Signalling failure?
8. Why do trains have signals?
9. What is fixed block Signalling?
10. What do train lights mean?

Tags

1. #control
2. #moves
3. #shunting
4. #opposed
5. #signals
6. #those
7. #subsidiary
8. #train
9. #movements
10. #which
11. #usually