With a flat face insert, there are still 3 "axes" of rake. 1.) Axial: looking at the side of the tool stood shank vertical, does the insert lean front or back at the top, or neutral? 2.) Radial, (looking straight on at the end of the tool, does the line described by the cutting edge of the bottom of the flat insert intersect above center of the axis of rotation of the tool (negative), below (positive) or neutral? 3.) "lead" angle: Looking at a shadow of the tool held vertically, does the insert cut a square shoulder, or an angled shoulder on the work? I can't think of any undercut leads, except dovetails. Generally the lead angle is "negative" though typically described as a positive angle of 90deg or less. E.g., a 75 deg lead angle will leave an angle at the edge of the cut path of with that included angle, or 15deg off vertical.
Tools have a combination of all three, designed to accomplish a certain package of "benefits" for a given application. A moderate lead angle "softens" the cut by spreading the impact of an interupted cut or edge entry over a larger area of edge and creating a slicing effect (shear) So a "negative" lead, creates the effect of a positive cut. Too much may give a lifting effect that overpowers the available rigity of smaller machines. OTOH, the tool may have a more universal application, if it creates a 90deg edge, and sacrifices some smoothness of operation.
edited: Obviously my description best fits rotary cutting tools such as end mills/face mills, etc. But the 3 axes of orientation of a flat faced cutting edge to the work applies as well to tools held stationary while the work moves, such as planer or lathe tooling.
smt