What does it mean?
A quantity takeoff is the process of extracting quantities from drawings, models or site measurements. Lengths, areas, volumes and counts are recorded with enough references that another estimator can check them. It is a measurement exercise before it becomes a pricing exercise. Drawing revisions and dimensions are part of the record, not optional background information.
In practice
For a rectangular slab, record the drawing reference and multiply 6 m by 4 m by 0.15 m to obtain 3.6 m³. Keep openings, thickened edges and wastage as identifiable adjustments.
Keep in mind
For a project record, note the document reference, revision and agreed measurement or contractual basis beside the quantity. Keep assumed information visibly separate from confirmed information. If two parties use different definitions or inclusions, resolve that difference before comparing amounts. These explanations are general estimating context; the wording of the applicable contract and its administration process must be checked for the particular job.