Defence listed in EFT is raw damage i.e. the figure you get by multiplying the weapon damage modifier by the ammo damage values, and then dividing by the cycle time. The firepower dps figures are also raw figures and are reality lessened by your targets resists.

If your setup is passive or cap stable then sustained and reinforced will be identical.

If your setup is not cap stable then reinforced is how much raw dps you can tank by pulsing you repper module in such a way as to avoid capping out. Sustained is how much dps you could tank by constantly repping until your cap was drained.

Example. Your ship has 50% EM armor resist and your amour repper does 320 hp every 9 seconds. Your repper does 35.5 hp per second and if you were being hit with pure EM damage you could tank 71 dps sustained.

With a second repper (if cap stable) you would have a 142 dps tank - however if you can only perma-run 1 of the reppers and had to pulse the second your reinforced rating would be in the region of 120 dps depending on how close to cap stable you where, cap size, cap regen etc...