Special Keys
RMK maps all keys QMK does. However, at the time of writing, not all features are supported.
The following keys are supported (some further keys might work, but are not documented).
Repeat/Again key
Similar to QMK, pressing this key repeats the last key pressed. RMK binds this function both to the standard Again keycode and to a dedicated Repeat special key. Binding Again ensures better compatibility with Vial, which features the Again key as a dedicated key (unlike the RepeatKey, which doesn't exist in Vial). Although some old keyboards might have a key for Again, it is not used in modern operating systems anymore.
In QMK an AlternativeRepeatKey is supported. This functionality is not implemented in RMK.
Caps Word
RMK includes CapsWordToggle (case-insensitive, no aliases). Caps word capitalizes letters (and turns - into _) until you press a key other than a letter, digit, -, Backspace or Delete, or after 5 seconds without one of those keys.