Soldiers need to be able to communicate on a noisy, dangerous battlefield even when conventional means of communication are unavailable.
To help meet that need, the US Army has just updated its compilation of hand and flag signals.
One configuration of flags signifies “Chemical, biological, radiological, and nuclear hazard present”:
Or a soldier may need to signal “I do not understand,” as follows:
See Visual Signals for Armor Fighting Vehicles (Combined Arms), GTA 17-02-019, US Army, February 2018.