Some basics on the hierarchic structure in Falcon:
Oscillator – Keygroup – Layer – Program – Multi
Keygroups can contain numerous oscillators (also of different types and with round robin chains), the mapping is done on keygroup level. Layers can contain multiple keyroups, Programs can contain multiple layers (also with round robin chains and plenty of other rules defined in layer rules). Multis can contain multiple programs.
Polyphopnic/per voice modulation are only possible on oscillator and keygroup level, on layer level they become monophonic (there is a workaround for some special situation). All filters and some effects can be inserted on keygroup level, most effects and can only be inserted on layer/program level (e.g. you can’t have a reverb unit per voice as that would bust any CPU) but you can use e.g. a waveshaper on keygroup level.