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According to the documentation
If other rules already exist for the selected ports, then those existing rules take priority over the new "deny all inbound traffic" rules. If there are no existing rules on the selected ports, then the new rules take top priority in the NSG and Azure Firewall.
So, the new rule always will assign the top priority to the selected port, however, if you already created a rule for a port, your rule will be prioritized.
Hope this helps!
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