In what order are users listed under Contacts?
Contacts are listed in the order of usage based on mails sent in Zoho Mail and messages sent in Cliq or Chat Bar.
If you send just one message/mail/call to a user, they will get a usage count of 1 that day. So based on the highest usage count, the users will be listed under Contacts.
Say for example Scott sends an automated email daily to Olivia, and chats with Ryan frequently but not daily. So Olivia's count will be higher because a mail is triggered everyday whereas Ryan's count gets updated only on days chat/call/mail is established. Hence Olivia will be listed on top in Scott's Contacts followed by Ryan.
And this order will be retained for 90 days, post which the process will start again thereby removing inactive users from the list.
This is the same order that will be populated everywhere for you – users listed for @ mentions, add participants view in group chat, frequent contacts in search box, ...