Here it is straight from the series 6 manual
Do not subject modules to high risk shading instances listed below: ► Resting or adhering slender objects (tools, brooms, clothing, wires, tape) on front-side of operating modules, or when within ~2 m (~6.6 ft) above operating modules, especially when the shadow is oriented parallel to cells ► Fixed objects within ~2 m (~6.6 ft) above operating modules that cast a shadow over the long dimension of the cells. Close objects (posts, ropes, fences, etc.) can begin to increase risk of partial shading of full cells when within ~2 m (~6.6 ft) from the front-side of operating module ► A support frame or mounting method on the long edge(s) of modules that fully shades the entire length of a cell (either partially or completely) ► Cleaning robots or other mechanisms that traverse the module while the system is operating ► Row-to-row shading when the modules are installed in landscape orientation ► Closely “stair-stepped” trackers on northerly slopes (northern hemisphere), or southerly slopes (southern hemisphere).