All policies within a domain are mutually exclusive and are prioritized (ordered) by the administrator. Only one policy will be applied for each message for a given domain - the first policy that matches a given message will be applied to that message.
If a message contains characteristics which meet the requirements of multiple Retention Policies, the Archive service will use the order in which your policies appear on the Retention Policies overview page to determine the policy which will be used to Archive that email, with the number policy listed as number 1 taking first priority, followed by number 2, then number 3, etc.
Here is an example. Your domain's first Retention Policy is called "Invoices" and it is configured to retain any email with "Invoice" in the subject line for three years. Your domain's second Retention Policy is called "Large messages" and it is configured to retain any email over 5 megabytes in size for one year. If the service receives a 10-megabyte email with a subject of "New Invoice", that message will be archived for three years based on the subject line match of the higher priority "Invoices" policy.
Administrators can re-order a domain's retention policies at any time via the Retention Policies overview page, by dragging the individual policies up or down to determine the relative priority of each policy.