If a number is written as, for example, 317.828 38 (the mass of Earth relative to the mass of Jupiter), what does that mean?

just take a dink man, it looks like ur working too hard.

actual value according to wikipedia is 317.8 Earths

I suspect that number means

317.828 ± 0.038

which is sometimes written as

317.828 (38)

(I regard this as somewhat ambiguous)

317.82838 is too many digits beyond the known accuracies.

It means there has been a typographic error. Or, the number is written in the European style, with a thin space after a group of three digits. In the Jupiter/Earth case, the ratio is known to be 317.828383, plus or minus 34 in the last two digits. So I vote for the "European style" explanation.