Slope(plyFile, Guess = F)
In the case of applying this function to teeth (its intended purpose) the function expects a surface with the occlusal plane normal to the Z-axis.
The 'Guess' parameter is a logical asking whether or not you want the function to both guess as to the right side up of the surface, and to then discard all of the 'negative' slopes i.e. surfaces which are over-hangs, as is frequently found on the sidewalls of teeth. If 'Guess' is not engaged the mean slope will include the negative values of the overhang and will likely underestimate the average slope of the surface.
Regardless of if the 'Guess' parameter is engaged, the function will also return a vector containing all of the face slope values ("Face_Slopes")