Step 0: This is not necessary when on a firewall running SonicOS 7.0 or above.
Go to Firewall Settings -> Bandwidth Management and change the Type from None to Advanced.
Step 1: Create a service object, type TCP, ports 9081 and 9082. Those are the two ports Axcient backup traffic uses.
Step 2: Create a bandwidth object. Set it to be whatever you want the maximum usage of Axcient to be - this will be different for each client, RUN SPEED TESTS AND CHECK THEIR UPLOAD AT PEAK TIMES FIRST. Set the traffic priority to Low.
Step 3: Go to the firewall Access Rules, and create a new rule.
Source: LAN (or whatever zone your clients are in, one rule for each zone)
Destination: WAN
(Destination) Service: Axcient
Make sure it's set to Allow, then switch to the BWM (6.0) or Traffic Shaping (7.0) tab.
Check the box for Egress bandwidth management and set the object to Axcient.
Check the box to enable tracking, then hit Add.
Enabling tracking will allow you to view in real time what percentage of the bandwidth allotment Axcient is using.
If you need to adjust later you can simply change the value for the bandwidth object up or down.
(Also bear in mind more specific rules take precedence over general rules, and Sonicwalls are capable of schedules, so you can create specific rules targeting, say, the servers, if they have different requirements and need to take priority over this rule.)