The number is the yield strength in newtons per square millimetre; the D is the ductility class. Fe 500D bends and absorbs shock better than plain Fe 500 at the same strength, which is why seismic codes lean on it. Fe 550D is stronger again, and lets a designer use less steel for the same load.
Higher grade is not automatically better value. It only pays when the design actually uses the extra strength, and a mixed site that substitutes grades without redesigning is buying an expensive bar for no benefit. Specify the grade on the inquiry so every seller quotes the same thing.
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