You can edit and play your game at the same time, and most of your edits come into effect instantly. This feature is unfinished, so many of your edits will have no in-game effect (see the lists below) and there are lots of caveats:

-"Test Game" does NOT save your game! You still have to do that in the normal way!
-Reloading will not happen while you are in most builtin menus in Game (e.g. `Status').
-Game can't reload data until Custom saves it. As a general rule of thumb you'll either need to exit whatever editor you're using in Custom for changes to be saved, or change record (e.g. switch to a different enemy, hero, textbox...)
-If Custom freezes, try quitting Game, and vice-versa.
-Resizing a map while you're on it is a bad idea; it's not handled correctly.

You can press Ctrl+S in the map editor proper and maptile editor to force immediate saving of data. While playing, press the F5 debug key (currently only works on maps) for lots of options about reloading map state or scripts.

Instant reloading of the following things currently works:
-All map data, including NPCs (also vehicles), general map settings, and doors
-All graphics, and tile animations
-16 color palettes, the master palette, and UI colors & box styles
-The font
-Music and sound effects
-Attacks, as long as they haven't been queued already (?)
-Preference bitsets and other General Game Data
-Unequipped items (stat bonuses and the like are read from file when you equip them)
-Scripts which aren't running (see below)

The following things will reload, but normally only after leaving and re-entering a menu or battle:
-Enemies (leave and re-enter the battle first)
-Formations (ditto)
-Menus
-Shops
-Global text strings

The following things do not currently reload at all:
-Hero definitions for heroes which are already in the party (this is a problem with saved games too!)
-Default palettes (which palette is the default for a sprite)
-Stat bonuses for equipped items

Loaded/running scripts can't be reloaded. Once they exit and are re-triggered the new version will be loaded. This means a mix of old and new scripts can run at the same time! Use the F5 Reload menu to disable automatic reloading.