Nothing.
It just happens to be the largest digit and that is probably because we count in ten and that, in turn, is because humans have ten fingers (including thumbs).
The divisibility rule for 9 etc would work with any base. The number 7 in octal, or F (=1510) in hexadecimal have the same properties.
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Digits are in alphabetical order: Eight, Five, Four, Nine, One etc
The nine is in the ones place.
Ah, isn't that a lovely little number? To write out 9.9 percent, you simply write it as "nine point nine percent." Remember, each number has its own special place and deserves to be written out with care and attention. Just like when you're painting happy little trees, take your time and enjoy the process.
Nine People
The largest 9 digit number is 999,999,999 (nine hundred and ninety nine million, nine hundred and ninety nine thousand, nine hundred and ninety nine).