In the real world tactical community, your "secondary" is the weapon you transition to when your primary goes dry. A secondary is a pistol type weapon. When you have a sniper rifle and an M4 (or other "long rifle") the M4 would be considered an "alternate" or "backup". If you carry two pistols the second pistol (when one is your "secondary") is also considered a "backup", but is also sometimes known as a "tertiary" weapon.
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