What is “Support Class” in the Character Editor?
Short answer: set it to the class the unit starts as for player units and bosses, doesn’t matter for generics.
The actual mechanical effect the “Support Class” field has in-game is mainly just determining what class the character is displayed as in the support viewer menu. It may also be used as a default if you don’t specify a class to load a unit as, but I’m not certain on that, and wouldn’t advise doing so when you can just specify the class in the unit placer.
The real reason you should always make sure to set this for player units and bosses is for your own convenience with setting stats. FEBuilder’s character editor includes the ability to check a character’s average stats at a given level, which can be used to view base stats by checking them at their starting level. A character’s base stats when they’re actually loaded are their personal base stats plus their class base stats. The “Support Class” is what FEBuilder uses to determine which class’s bases to factor into the averages it shows you in this tool.
What is “Menu Class” in the Class Editor?
“Menu Class” is largely a leftover from FE6 and FE7, where it was the field used to determine what a class promotes into. In FE8, this is instead handled in the Promotion Branches editor under Advanced Editors.However, Menu Class does still have an effect on EXP gain for promoted classes, as explained here – in short, don’t leave it undefined for promoted classes, or their EXP gain will get messed up. By convention, it’s usually set to the promoted/unpromoted counterpart of the class.
Menu Class is used by the patch that allows three-way promotion branches as the way the third promotion choice is stored.
Menu Class does not have any effect on the stats of promoted enemy classes.