Size charts, spec charts, and grade rules are all terms that are thrown around a lot during development and pre-production. While they do sound (and sometimes look a bit) similar, their meanings are all distinct and each has a different use and purpose.
The spec sheet is an important part of the tech pack that shows what the style measures at each POM in all 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.