Command Meld

Command Meld is a system in Kingdom Hearts Birth by Sleep that replaces the traditional Item Synthesis. Command Meld allows the character to create new Commands by combining two previously obtained commands; the resulting Command will differ based on the types and level of the ingredient Commands. Furthermore, synthesis materials, such as the Fleeting or Abounding Crystal, can be added to the mix. This does not affect the resulting Command, but instead attaches a certain Support Ability to the Command, depending on the material and the final Command. Like in Final Fantasy games, this ability will be equipped with the Command, and will level up as long as the character gains experience. Once the Support Ability gains maximum experience, it will be moved the character's permanent Abilities list, allowing them to use it no matter what commands they equip.

For example, mixing a Lvl 1 Fire Blitz and a Lvl 1 Aero will produce Aerial Slam. If a Bright Crystal is added to the mixture, the Aerial Slam will have an attached HP Up ability.

Trivia
Like most other gameplay features in Kingdom Hearts Birth by Sleep, Command Meld is similar to a gameplay feature in Crisis Core -Final Fantasy VII-, known as Materia Fusion.