What causes Microsoft Outlook rule to be client-side only?

I can clearly see and confirm that adding and mark it as read flag to any rule causes my Outlook to start treating given rule as client-only:

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While removing this flag causes that particular rule will be effective also in Outlook Web Access (at least that's I assume from these warnings):

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So this is the ultimate answer to my question: Forcing any rule to mark any message as read causes that rule to work only, if offline Outlook program is opened.

However I completely don't know, why is this happening like that? Why simple operation of marking message as read can't be processed in Outlook Web Access. Must be a weird part of the a matter deeply in Outlook.

Server-side vs. Client-only Rules:

Client-only rules have at least one condition or action that uses an Outlook feature:

  • Common conditions making it a client-only rule:
    • Assigned to category
    • Flagged for action
    • Marked as importance
    • Marked as sensitivity
    • With specific words in the body
    • With specific words in the message header
    • With specific words in the recipient’s address
    • With specific words in the sender’s address
    • With specific words in the subject

  • Common actions making it a client-only rule:
    • Assign it to the category
    • Clear the Message Flag
    • Display a Desktop Alert
    • Display a specific message in the New Items Alert window
    • Flag message for follow up at this time
    • Mark it as read
    • Moving messages to a folder that exists only on the PC
    • Permanently delete it
    • Play a sound
    • Print it

  • Outlook also imposes a storage limit of 32KB for rules in OWA