Answer to b
It is used to quantitatively describe the retinal topography 
and the follow-up topographic changes.

The typical application of the HRT is the assessment of the 

glaucomatous optic nerve head.In the case of optic nerve
head analysis, after the definition of the disk margin the HRT 
software computes a set of stereometric parameters that 
quantitatively describe the shape of the optic nerve. The results 
of the topographic description are used to classify an optic nerve
head as being normal or outside normal limits. Furthermore, the 
stereometric parameters are used to describe glaucomatous progression.