Seal (封じ Fūji) is an official term which refers to losing the abilities of a unit, as well as the main mechanic of the "Exorcist Mages". In Cardfight!! Vanguard ZERO, Seal is treated as a keyword ability and it works slightly different.
How It Works
By choosing one specified ability from any of your units, that ability is lost and does not exist, until end of turn. It allows to the affected unit to be freely used, such as "Lightning Sword Wielding Exorcist Knight".
By choosing any of your rear-guards, it loses all of its original abilities for the rest of the game for as long as it is on the field. Sealing a unit does not remove abilities that were previously granted or are granted later.