Citation 2 +1 FL in LIDO means that the system recalculates the entire flight plan on the same lateral route using one "usable" FL above or below and provides the time and burn difference.
One usable FL means that LIDO looks at each flight segment from waypoint to waypoint and adds/subtracts the correct FL based on the airspace rules for that particular area i.e. RVSM or non-RVSM, FL in feet or meters or the combination of both.