If you walk into a store and ask for 60/40 solder, that is what you're going to get.
If you walk into a store and ask for 63/37, they'll probably hand you 60/40 and say this is all they have and it's close enough. And for most things, it is close enough. But 63/37 is different than 60/40 and will be marked as 63/37.
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