There are no implicit transactions in ANSI session mode. More accurately, each ANSI transaction is initiated implicitly, but always completed explicitly. A COMMIT or ABORT/ROLLBACK request must always be explicitly stated and be the last request of a transaction in order for the transaction to terminate successfully.
In ANSI session mode, you must issue an ABORT/ROLLBACK (or COMMIT) request even when the only request in a transaction is a SELECT or SELECT AND CONSUME.
For a SELECT request, there is no difference between issuing a COMMIT or an ABORT/ROLLBACK.
For a SELECT AND CONSUME, there is a difference in the outcomes between issuing a COMMIT or ABORT/ROLLBACK because an ABORT or ROLLBACK request reinstates the subject queue table of the request to its former status, containing the rows that were pseudo-consumed by the aborted SELECT AND CONSUME request.