There is more information in the README that might be part of the distributed package, or can be seen here 3.8 Pseudo file support.
-p 'mychardev c 666 root root 100 1'
creates a character device with major/minor 100/1.
Similarly, if you have a file
mylist holding the lines
mydir d 777 0 0
mydir/thedate f 776 0 0 date +'year is %Y'
-pf mylist will create a directory and a file within it holding the result of doing the command
date +'year is %Y' at the time
mksquashfs was run.
-sort option is not described further, but accepts filenames within the resulting filesystem, followed by a number, for example
b/d are found in your squashfs. My version does not recognise names provided through the
-p options, and indeed stops the above
date example from working. If necessary you can split up the building of the filesystem into separate
mksquashfs commands with different options, and each set of new files will be appended at the end.