"3.7.1. If during a battle an unsafe situation occurs, it is the responsibility of all Combatants (and Marshals) who see the situation to call “HOLD” and stop the battle. A HOLD stops the battle while the Marshal assesses the situation. The battle resumes only at the Marshal’s discretion." - Book of War
When the term "Hold" is yelled across the battlefield, all fighters should join in on calling hold until everyone has stopped fighting and all fighters have taken a knee. The "Hold" command is designed to end combat so medical personnel can enter the battle area or to prevent combat from entering an area that is not safe.
Calling a Hold
If someone is injured or a serious safety issue is discovered, a fighter should call "Hold". Because of this the term "Hold" should not be used on the battlefield except to stop combat because of an injury.
When not to call a Hold
You should be careful about when a hold is called. Someone being knocked unconscious is a good reason to call a hold. But just because someone receives an illegal strike to the face is not a valid reason to stop combat.
Avoid using any phrases with "hold" such as "Hold the line." or "Hold your ground." They end up confusing people and causing them to stop combat even when it is not necessary. Better phrases might be "Stop moving." or "Stand your ground." Relatedly, avoid having team names that sound like "hold" such as "old", "gold", "cold", etc. Even words as seemingly unrelated as "four" have been known to cause problems.
Another common mistake is using the word "hole", such as "fill this hole in the line!" A good alternative is "fill this gap!"
Bottom line, always be mindful of what you are yelling across the field.