The admin checkboxes control mostly what SITE commands a FTP client can issue to the server. These SITE commands have specific features they do.
VERSION - gets you the current server version.
USERS - lists currently connected sessions.
KICK (session id) - use the USERS SITE command to get the session list.
KICKBAN (session id) - use the USERS SITE command to get the session list.
PASS (password) - allows you to change your password.
ZIP - makes all dir listings end everything with a .zip extension so a GUI client can understand the list and download an item as if it were a .zip file.
DOT - shows or hides items starting with a dot as the first character.
QUIT - kills the CrushFTP server as soon as this user logs out.
CHMOD 777 (filename) - changes the Unix permissions on a file.
CHOWN (user) (filename) - changes the unix owner on a file.
CHGRP (user) (filename) - changes the unix owner on a file.
Allow passwords to be emailed - controls if the login page of the WebInterface will email this user their password.
Allow Web Proxy / Adhoc proxy - controls if a built in proxy mode CrushFTP has can be used by this user, and if they can generate their own proxy connections.
Remote Administration - allows this user to connect with another copy of CrushFTP to do remote administration of the server. The everything gives them full access to all settings, while the limited option gives them settings only to the user manager, and users you have configured for them to access.