Secondary Weapons are used as backup weapons. They must be carried with Primary Weapons.
To switch between the primary and the secondary weapon you must press Q or SHIFT.
Most of them aren't as effective as Primary weapons because of lower firepower. They are mostly used when the Primary weapon runs out of ammo, or just to make up for its weaknesses. There are 3 types of Secondary weapons. All classes can use only 2 types, but the Engineer and Elite can use all 3.
Except for Blades, they need to be reloaded when the magazine runs out of ammo, and a Drum Mag increases the capacity to 210%, but does not remove the need to reload. However, they have infinite backup ammo, unlike primaries.
In the Capture The Flag Game Mode, a soldier can use only the Secondary while carrying the package/flag.
An AI controlled soldier will only use a ranged Secondary after running out of ammo for the Primary, but if equipped with a Blade, will always use it when in melee range of an enemy.
The Righteous Bizon and RCP 90 are exceptional secondaries, because especially when upgraded, thanks to their DPS, large magazine size and decent accuracy, can compete with many of the Primary weapons. This makes them a good substitute for weak Primaries and a good complement to short-ranged ones.
Blades are only melee weapons in the game. They usually have high damage but you need to come close to hit the enemy. They can be used by all classes except Gunslinger and Medic.The new Knight class can only use this weapon for both his primary weapon and his secondary as well.
Submachine Guns (SMG)
SMGs are mostly full-auto weapons with high rate of fire, but low damage and accuracy. They are replacement of SFH 2 machine pistols, which are now dual primaries. SMGs can be used by all classes except Sniper and Ninja.