The position in line where the output was produced. It is measured in
UTF-16 code units and the client capability columnsStartAt1 determines
whether it is 0- or 1-based.
If an attribute variablesReference exists and its value is > 0, the
output contains objects which can be retrieved by passing
variablesReference to the variables request as long as execution
remains suspended. See 'Lifetime of Object References' in the Overview
section for details.