On grammatical class:

fa'a
location tense relation/direction; arriving at/directly towards ...
to'o
location tense relation/direction; departing from/directly away from ...
ze'o
location tense relation/direction; beyond/outward/receding from ...
zo'a
location tense relation/direction; tangential to/passing by ...
zo'i
location tense relation/direction; nearer than .../inward/approaching from ...