When you make up a Shotlock, you'll decide what the Command Meld is for it and will need to know the two Abilities beforehand.

Sezir, Command Melding was a mechanic used in Birth by Sleep for learning and leveling up Abilities. It involves combining two Abilities you already know to create a new one more or less. Shotlocks were often techniques that were learned by this method. For example, Meteor Shower was potentially learned by melding Thunder with either Fire or the Blitz attack. Most Shotlocks have various recipes for learning them. In the RP, Shotlocks will be learned as Major Abilities, but to be able to learn them you must have already learned enough abilities for at least one of the Command Meld recipes. In the case of Meteor Shower, that would mean you'd have to at least learn the Thunder spell and, say, the Fire spell before being eligible to learn Meteor Shower later on.