When installing rails for a FlashArray in a rack requiring cage nuts (square hole racks that are not using the tool-less mechanism, or specific threaded adaptations), the correct placement for the cage nut is the top square of the lowest Rack Unit (RU) that the rail will occupy.
Pure Storage rail kits typically secure at the bottom of the targeted rack unit.
The "3-Hole" Rule: A standard Rack Unit (1U) consists of three vertical holes (often referred to as the bottom, middle, and top hole).
Alignment: The rail flange is designed to align such that the retaining screw or positioning stud engages specific holes to support the weight. For the standard securement point, the documentation specifies installing the cage nut in the top hole of the specific RU where the rail base sits (often the "lowest" RU if the device spans multiple U's, though the rail itself is 1U high).
Precision: Placing it in the "Lowest square of the lowest RU" (Option A) would interfere with the rail's shelf lip or the device below it. Correct alignment ensures the rail is level and the chassis slides in without binding.