Don's answer is good. The sourceid is what links the record in
Scriblio back to the ILS or other system it came from. A full sourceid
includes both the unique number from the source system, as well as a
two character identifier prefix that you assign that system.
The Flickr importer uses the ID number Flickr assigns to the photos as
the sourceid. The III importer uses the bibnumber from that ILS. An
example use is to import records from a number of III systems, using a
different prefix for each.
When records are re-imported, the sourceid is matched between them and
that prevents duplicates. And if the ILS needs to be queried for
location or availability information, the sourceid is the link between
them.