The FCC only requires a suffix in a few cases. If you have passed the test for a license upgrade and are using those new privileges, but they have not been registered at the FCC yet, you use a suffix. After I passed my Extra exam, I ID'ed as K6WRU/AE for a few days. The "AE" is for "acting Extra".
The rest of the rules for "indicators" are:
Each indicator must be separated from the assigned call sign by a slant ("/") or any suitable word that denotes the slant mark ("portable," "stroke," etc.). If the indicator is self-assigned, it must be included before, after, or both before and after, the assigned call sign (example "KP2/W1AW/contest"). No self-assigned indicator may conflict with any other indicator specified by the FCC Rules (such as "AA", "AG", "AE" or "KT") or with any prefix assigned to another country (such as "DL", "F", "G" or "VE").
"KT" is used for upgrade to the Technician license. "AT" would conflict with India.
Incidentally, this answers a previous question. Under FCC rules, it appears to be legal for me to ID as "portable/K6WRU/portable". Because no country has the "P" ITU prefix, I could instead have that before or after or both. "M" as a self-assigned indicator is not allowed, because that is an ITU prefix.