If the value entered after "@" (a half-width "at" sign) completely or partially matches any of the strings set for the following items configured for users, departments, and groups in Users & system administration, those users, departments, and groups will appear as mention suggestions.

If a user name is entered:

• Display name
• Localized name
• Phonetic first name and last name
• Login name

If a department name is entered:

• Department name
• Localized name
• Department code

If a group name is entered:

• Group name
• Group code

The display order of names in the suggestion list is determined based on the conditions listed in the priority order below.

• Whether there is a prefix match with the input string
• How many times you have mentioned the user in the last 30 days
• Users' display order set in Users & system administration
  Reference: Changing the display order of users