If you use an IMAP mailbox (which is recommended), and have already created some folders, it might happen that at some point after a newly created folder:
(only this computer)
This is due to the fact that (depending on the e-mail provider) usually only a certain contingent of so-called IMAP folders are provided by the provider. What does this mean when you create a new folder in Outlook then it will automatically be created in the folder structure on your e-mail server and you will have the sort order already on the e-mail server, not in Outlook.
Now if this quota is exhausted and you have a folder behind the “(only this computer)“, then the emails are still sorted according to your rules, but only within Outlook on your computer, and nothing more on the e-mail server.
This is always noticeable when you retrieve your e-mails with different devices, and you then lack the sorting there. Unless of course you set the same rules there again. Only halt finds no more synchronization between the devices.
See picture: (click to enlarge)