Q: Sometimes i have to login using AIT email with domain (@ait.asia or @ait.ac.th) but sometimes without domain (i.e. username only). How can I know which login is to be used?
A: If the application server’s URL is ended with AIT domain name (i.e. ait.asia or ait.ac.th), then it is a server owned by AIT, thus you have to login using username only. Example: sis.ait.ac.th, students.ait.ac.th, intranet.ait.ac.th, moodle.ait.ac.th, etc.

Other servers are not owned by AIT, then you have to login using domain. either a specific AIT domain or any AIT domain. Example:
gmail.google.com (use @ait.asia domain), portal.office.com and any Microsoft domain such as microsoftonline.com (use @ait.ac.th), turnitin.com (any domain if you do not use Google authentication).