The floor collects lateral force and delivers it to the walls — but a rigid slab and a flexible deck distribute that force in completely different ways.
A rigid diaphragm (like a concrete slab) distributes lateral force to walls based on their relative stiffness and position — offset walls create torsion. A flexible diaphragm (like a metal deck or plywood) distributes force by tributary area, like a simple beam spanning between walls. Toggle between the two, move walls around, and watch how the reactions change.
A diaphragm is the floor or roof plate acting as a horizontal beam. Wind or seismic forces push against the building face and enter the diaphragm. The diaphragm spans between shear walls or braced frames (its "supports") and distributes the lateral force to them. The edges of the diaphragm act as flanges (chords) carrying tension and compression; the interior acts as a web carrying shear.
A rigid diaphragm (concrete slab) distributes lateral force to walls in proportion to their relative stiffness — a stiffer wall attracts more force. A flexible diaphragm (metal deck without concrete, wood sheathing) distributes force based on tributary area — each wall gets the force from the floor area closest to it. Getting this classification wrong changes every force in the lateral system.