Collection UpsertAsync success

When we perform

   await collection.UpsertAsync()

we get


From current documentation it is not quite clear what is CAS. Can it be used as fail/success flag for upsert? if yes, what is the value for fail?
If no, how can we identify success/fail for upsert?
Thank you

SDK 3 uses exceptions for error cases, so as long as there are no exceptions then it was successful. CAS is the Check-And-Set Compare-and-Swap, which is used when reading and then mutating a document to ensure it wasn’t changed between the read and the mutate.