Casual and Classic modes in the game mode settings in Awakening
Classic Mode refers to the traditional gameplay mode used in the Fire Emblem series since Fire Emblem: Shadow Dragon and the Blade of Light. This mode was officially given a name in Fire Emblem: New Mystery of the Emblem in Japan and in Fire Emblem Awakening for international audiences.
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The Fire Emblem series emphasizes the value and purpose of the playable cast by giving them meaningful characterizations, appearances, and in-game abilities and trends unique to each character within the game to foster closer bonds with the cast. Due to the tragedies of war, the series further emphasizes the weight of death and loss of such valuable allies. Since the beginning of the series, should a character of the player's army lose all of their HP during the course of a battle, they are permanently retired from battle with a vast majority outright dying within the game's narrative. Once this happens, that character can never be used again for the rest of the current save file's story progression.
There are two exceptions towards character death within the series. Some characters can be permanently retired from battle but only because they sustained grievous injuries that permanently benches them unless they are needed for cutscene dialogue. Even though said character survives the story, they either die shortly after in the credits and epilogue of the game or are simply treated with a "retired" label and given a much more simplistic ending. In Fire Emblem Gaiden and its Remake, character deaths can even change the epilogue of other associated characters that did survive such as Clive and Mathilda or Mae and Boey.
The other example are the deaths of the Lord, the player's Avatar, or a necessary NPC that needs to survive a battle. The former two examples are a frequent Defeat condition in nearly all battles, forcing the player to have to restart an entire battle from scratch from the last save point as these force a Game Over or force the Turn Rewinding mechanic to activate if available. Due to this all Lords and Avatars are the only units in Classic Mode that must survive the game in order to complete it.
Some games do have some safeguards or fall back to ensure that a player is not completely penalizes for certain undeterminable factors attributed to luck. Fire Emblem: Radiant Dawn and Fire Emblem: Shadow Dragon feature save spot tiles which allow the player to resume/restart a map from them mid-point of the battle. Some games also feature a revive mechanic of sorts, usually done by a legendary Staff. The Aum Staff, Valkyrie Staff, and Bifröst are some examples of such staves. To limit these powers, they are limited to a single Usage and requires high proficiency with staves. They have no limitation on which character they revive, though they revive said character at the level they were at the time of their deaths. These staves also generally require the character using the staff to have the highest Weapon Level proficiency in Staves and are usually acquired in the final chapters of the game. Gaiden/Shadows of Valentia have Lion Head Statues in several Dungeons that have the power to resurrect a fallen ally. However, they have limited usage as well with each resurrection Sacred Springs having three charges each. Gaiden has two of these Sacred Springs, making six total usages, while Shadows of Valentia has three, totaling nine uses.
Since Fire Emblem: New Mystery of the Emblem, alternative play modes have been introduced to ease players into the series or provide a more casual approach to the series' Classic Mode. Dubbed Casual Mode, this mode allows players to use most characters again after battle as, instead of outright dying, a character retreats from battle after sustaining enough damage and makes a full recovery by the start of the next battle.