When it gets low or reaches zero then you can purchase additional signal credits with «top - ups» that start at $ 5 for 500 signals. (daytradingcoach.com)
Did language arise in the attempt to communicate about signals and to invent additional signals, gradually developing beyond signals into genuine symbols? (religion-online.org)
Additional signal effect is achieved by a reflective luggage grid with foil and other indicators. (topspeed.com)