Carnage Unity is the process of taking materials dropped from Mirages and turning those into Carnages, or weapons. Different weapons become available depending on what class a certain Mirage is. For example, Chrom as a Great Lord has access to , while as a Conquerer he has .
Weapons are not exclusive to a class, however. After unlocking them by changing a Mirage's class, they are free to use from then on. It is possible to change Chrom into a Conquerer for Balmung, then still be able to use it should he become a Great Lord afterwards, and vice versa.