LEDs per metre tells you how dense the phosphor line is. It does not tell you how much light leaves the strip. Two boards at 480 LEDs per metre can sit at 8 W/m and 10.5 W/m, and the fixture-level result follows the wattage and the bin, not the count.
Watts per metre sets the heat load you have to move. In a closed aluminium profile, a 14 W/m strip needs more surface than an 8 W/m strip of the same density; if the profile cannot shed it, output drifts. Check the wattage against the extrusion before specifying the density.
Cut length decides where the run ends. A 38.46 mm interval leaves a different remainder on a 4,020 mm cove than a 50 mm interval, and a 20 m single-feed 48V board removes the booster that a 12V reel of the same density would need.
Related series
Single colour