“An item that promotes a lesser-class unit at level 10 or above to a higher class.” —In-game description, Shadow Dragon and New Mystery of the Emblem
The Master Seal is a promotional item that recurs across the Fire Emblem Series, indiscriminately permitting all first tier units Level 10 and above to be promoted into their respective second-tier classes, save for those with scripted promotions.
In Fire Emblem: The Blazing Blade, the Master Seal is known as the Earth Seal (地の刻印 Chi no kokuin), in contrast to the Heaven Seal, which is used to promote Lords. The item does not promote characters of the Pirate or Thief class lines, as the Ocean Seal and Fell Contract are respectively required instead. In more recent titles in the series, the Master Seal now also works on these classes.
The Master Seal is more useful in the games that utilise a promotion system where an entire spectrum of items exists, each one serving the purpose of promoting units in very specific class lines (Eg: The Knight Crest is designed to promote Cavaliers and Knights). In the games where promotions occur automatically (Path of Radiance and Radiant Dawn), promotion items are only used for promoting earlier than Level 20. Since early promotion leads to lower maximum stats, doing so is not very useful in the long term, although it can serve as a sort of quick-fix when the player desires a specific unit to promote immediately and not have to spend time and resources building experience in order to nurture them to reach Level 21. Alternatively, one can use Master Seals on Level 20 units in order to save experience, since promotion by level up causes the player to lose one full level of potential growths in the various key stats.
Master Seals are also included in the spin-off Fire Emblem Warriors as a form of rare Material for solely Crest creation, in which the rules of promotion are different by being simplified. To promote, a character must be at least Level 15 as well as having a Master Seal on hand, which allows the player to create that Unit's Surge Crest. Because the character's level does not reset on promotion and all stat gains are scripted, a Master Seal can be used at any time without missing out of any benefits.
In Fire Emblem: Three Houses, Master Seals are the localized name of the Superlative Level Exam Pass (最上級試験パス Saijōkyū Shiken Pasu). Master Seals are required for units of lower tier classes to promote into powerful Master tier classes through the Certification Exam. Master Class can only be unlocked once Byleth's Professor Level has reached Rank C.