“For some units above Lv.10 who have not changed class.”
—In-game description, Rekka no Ken
“An item that promotes a lesser-class unit at level 10 or above to a higher class.”
—In-game description, Shadow Dragon and Shin Monshō
The Master Seal (マスタープルフ Masutā purufu, lit. Master Proof) 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: Rekka no Ken, 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, and the Ocean Seal and Fell Contract are respectively required instead. In more recent titles in the series, the Master Seal 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 causes the player to lose one full level of potential growths in the various key stats.