The spec sheet is an important part of the tech pack that shows what the style measures at specific places (called points of measure or POM) in each of the sizes offered. The chart also shows the tolerance for those measurements – meaning the amount plus or minus that the garment can differ from the spec measurement and still be considered acceptable.
These are garment measurements, not body measurements. These are the measurements that both you and the factory can use to confirm that each garment meets the spec you requested.
An accompanying POM illustration shows where each point is measured on the flat garment. Again, a good tech pack describes details in both words and pictures. The letters correspond to the measurements above.
For you as the designer, the spec sheet helps you with quality control your incoming samples and production runs. Using the chart and the illustration, you can measure your samples to make sure they meet the spec.
The chart can also help you write your product descriptions for your customers. You can supplement your size chart (body measurements) with detailed fit notes such as the garment measurements for total length, inseam, or rise. That way they can decide which is right right size to order.