The cones shape improves vertical angle pointing and the white color allows an instrument’s crosshairs to be more visible and center more precisely. The site cones also double as tilting-axis lock knobs.
The prism assembly features an adjustable 40-minute vial mounted on the front side; it’s easy to see and adjust. The prism can be set at 0 and -30 mm-offset positions; it measures 100 mm from point to tilting axis, and it can be handheld or pole mounted.
Source: SECO, Oct. 3, 2003