A double weapon has two different weapons placed end to end so you can attack with either easily without changing your grip. For the purpose of the Multi-Weapon Fighting feat, a double weapon is treated as two or more operative melee weapons. A double weapon is not treated as an operative weapon for any other purpose unless it has the operative weapon special property.
Some double weapons have ends that deal different damage types. When making a single attack with such a weapon, you can choose which damage type to deal, but if you make more than one attack in the same round, at least one of those attacks must be made with the second damage type. The weapon category of a double weapon that deals more than one damage type is based on the first damage type listed. If its second damage type causes it to be considered a different weapon category when dealing that damage, that category is listed in parentheses. For example, a double weapon in the flame category that deals 1d6 fire damage or 1d6 cold damage lists “double (cryo)” to indicate that when it is used to deal cold damage, it is treated as a weapon in the cryo category.