Meaning of "exec 9<name" in scripts

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Meaning of "exec 9<name" in scripts

I have found code like

exec 9<filename

a number of times when looking over commonly used scripts here. What all does this do? Sometimes the filename is simply a list, but seems to always have read/write/execute attributes for all. I think the "<" means to accept this as input, but don't know about the 9.

# 2  
It opens a file descriptor (9 in this case) for reading, from file filename.
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# 3  
Standard input is file number 0, standard output is file number 1, and standard error is file number 2. Any other number can be used for whatever purpose you want -- in this case, they attach a file to it.
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# 4  
And, somewhat further down the script, you might find a read -u 9 (bash) or similar to actually input values from that descriptor.
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