Are there units associated with SUMA's xyz coordinate system? I.e., is the coordinate (1, 1, 1) a 1mm movement in all directions from the origin?
Yes. It's what we call RAI (Right-to-left, Anterior-to-posterior, Inferior-to-superior). The values increase in the direction of those axes, so the starting sides of each axis will often be negative for MRI data, and the ending side will be positive. Much of AFNI, including SUMA, uses the RAI (DICOM) coordinate system. Note other softwares may call RAI as LPS (the larger sides of the axes). Coordinates may be more consistently reported with any system if the side of the axis is reported (e.g. "21R, 14A, 5S").