SASU differs from SMARTnet in that you will not be able to get advanced hardware replacement with SASU. I didnt know that SASU was even available for the UC520 but I would imagine that it would cover software updates and support of all of the components included with the UC520 (CME, IOS, CUE, etc..) but won't cover you if there is a hardware failure of the UC520.
As far as ordering goes, are you a Cisco partner or an end user? If you are a partner you will want to build the quote on the SCC site and then purchase through distribution. http://www.cisco.com/go/scc.
If you are an end user then I would recommend getting in touch with a local Cisco partner to take care of you.
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