On grammatical class:

ca'u
location tense relation/direction; forwards/to the front of ...
ga'u
location tense relation/direction; upwards/up from ...
ni'a
location tense relation/direction; downwards/down from ...
ri'u
location tense relation/direction; rightwards/to the right of ...
ru'u
location tense relation/direction; surrounding/annular ...
ti'a
location tense relation/direction; rearwards/to the rear of ...
zu'a
location tense relation/direction; leftwards/to the left of ...