It is not recommended to change the ACLs of these pre-made directories. If you need something different, make a sub-directory. The directory can be accessed only by yourself and the ICTP web and backup servers. If you accidentally delete a file or make unwanted modifications to it, here's the place to look for rescue.
ARCH contains platform specific bin directories.
The symbolic link bin in your home directory always points to the one corresponding to the computer where you are logged on. If you want some files to be accessible to a certain group of people, create another directory inside public and change the ACL accordingly.