If the value entered after '@' (a half-width at sign) matches or partially matches any of the strings set for the following items configured for users, departments, and groups in Users & system administration, these users, departments, and groups appear in the suggestion list of the Mention feature.

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