| the difference is 1/16th of an inch
a 520 chain is 5/8" by 2/8"
a 525 chain is 5/8" by 2.5/8" (5/16")
another common size is a 530 chain
a 530 chain is 5/8" by 3/8"
So the chain pitch is the same for all 3 but the sprocket width is different
520s are often chosen for racing as the narrower chain and sprockets are lighter, the 520 is standard fitment for the DD bikes where as 525 chain and sprockets are usually found on the 748/916/996 etc
Not sure if any bikes these days use the 630 chain that my old Kwak Z1000 used and my Yam R6 cam with a 532 chain as standard which shares the same dimensions as a 530 chain but with bigger diameter rollers.
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