F-SECURE ANTI-VIRUS FOR MICROSOFT EXCHANGE 8.00 Administrator's Manual page 234

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The priority numbers are used to define the order in which the host
tries to connect servers. Virus definition updates are downloaded
from the primary sources first, secondary update sources can be
used as a backup.
The product connects to the source with the smallest priority number
first (1). If the connection to that source fails, it tries to connect to the
source with the next smallest number (2) until the connection
succeeds.
4.
Click OK to add the new update source to the list.