The sheet number shows the discipline and the type of drawing. An architectural floor plan and an electrical lighting plan have different letter prefixes.
Example: a gym equipment estimator opens the floor plan sheet for the gym. Then the estimator opens the interior elevation sheet that the plan points to.
Check the title block on each sheet for the scale, the issue date and the revision. Follow the references on the sheet to its details, sections and schedules. Record the sheet number next to every quantity you take off. That record lets someone else check your count.