What type of systems rely on light signals for commands?

Fly-by-light systems are designed specifically to utilize light signals for commands, making them a specialized technology in avionics. These systems employ optical fibers or laser technology to transmit information, which provides several advantages such as reduced weight, increased speed of data transmission, and immunity to electromagnetic interference. The reliance on light signals allows for more precise and reliable control over the aircraft's operations.

Other systems such as digital, mechanical, or general avionics may utilize various forms of signal transmission that do not specifically depend on light signals. Digital systems primarily rely on electrical signals to convey information, while mechanical systems function through physical movements and mechanical linkages. Avionics systems as a whole encompass a wide range of technologies and signal types, not limited to light. Therefore, fly-by-light systems stand out as the correct answer in this context.