Hi, Lester. The SUID is a unique value you (as the developer) generate for each update.
From the Updates docs: "The SUID is one of the fields that identifies an update. A combination of the GUID, source, and SUID uniquely identifies an update. The SUID is an arbitrary and unique string generated by an application ... when the update is created." ("http://developer.yahoo.com/social/rest_api_guide/Single-update-resource.html#suidIdentifier")
The same doc page (above) also has a link to an example walkthru which shows how to create such a unique SUID.