What does # mean? - bash

Question

Here's the code I'm looking at:
#!/bin/bash
nc -l 8080 &
curl "http://localhost:8080" \
-H "Accept: application/json" \
-H "Content-Type: application/json" \
--data #<(cat <<EOF
{
"me": "$USER",
"something": $(date +%s)
}
EOF
)
What does the # do? Where is there documentation about #?

Answer 1

It is a curl-specific symbol. man curl shows you:
-d, --data <data>
(HTTP) Sends the specified data in a POST request to the HTTP server, in the
(same way that a browser does when a user has filled in an HTML form and
(presses the submit button. This will cause curl to pass the data to the
(server using the content-type application/x-www-form-urlencoded. Compare to
(-F, --form.
--data-raw is almost the same but does not have a special interpretation of
the # character. To post data purely binary, you should instead use the
--data-binary option. To URL-encode the value of a form field you may use
--data-urlencode.
If any of these options is used more than once on the same command line, the
data pieces specified will be merged together with a separating &-symbol.
Thus, using '-d name=daniel -d skill=lousy' would generate a post chunk that
looks like 'name=daniel&skill=lousy'.
If you start the data with the letter #, the rest should be a file name to
read the data from, or - if you want curl to read the data from stdin.
Multiple files can also be specified. Posting data from a file named
'foobar' would thus be done with -d, --data #foobar. When --data is told to
read from a file like that, carriage returns and newlines will be stripped
out. If you don't want the # character to have a special interpretation use
--data-raw instead.
See also --data-binary and --data-urlencode and --data-raw. This option
overrides -F, --form and -I, --head and -T, --upload-file.