After you've uploaded your documents and accepted your evidence, you may end up with a large number of approved items against a single grade descriptor. When that happens, tick Best Example to flag the one or two strongest pieces. You can find this tick in the Evidence Base when you open a piece of evidence that has been approved.
This carries through to your report: when you generate one, the spreadsheet shows exactly which items you've marked as Best Example for that descriptor.
Not every school uses it, but it's a quick way to help anyone reading the report find the standout evidence without having to sift through every item.