What are categories for "Construction Quality" and how is rating determined?

These are construction quality guidelines from the King County Assessor (credit SoCal_Engr).  They're pulled from the assessor's records.

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Classification by construction quality which refers to the types of materials used and the quality of workmanship. Buildings of better quality (higher grade) cost more to build per unit of measure and command higher value. (See Glossary in Area Report for Residential Building Grades in use by the King County Department of Assessments.)

Checking the area reports, which are available on the site but will cross your eyes, yields...

Grades 1 - 3
  Falls short of minimum building standards. Normally cabin or inferior structure.
Grade 4
  Generally older low quality construction. Does not meet code.
Grade 5
  Lower construction costs and workmanship. Small, simple design.
Grade 6
  Lowest grade currently meeting building codes. Low quality materials, simple designs.
Grade 7
  Average grade of construction and design. Commonly seen in plats and older subdivisions.
Grade 8
  Just above average in construction and design. Usually better materials in both the exterior and interior finishes.
Grade 9
  Better architectural design, with extra exterior and interior design and quality.
Grade 10
  Homes of this quality generally have high quality features. Finish work is better, and more design quality is seen in the floor plans and larger square footage.
Grade 11
  Custom design and higher quality finish work, with added amenities of solid woods, bathroom fixtures and more luxurious options.
Grade 12
  Custom design and excellent builders. All materials are of the highest quality and all conveniences are present.
Grade 13
  Generally custom designed and built. Approaching the Mansion level. Large amount of highest quality cabinet work, wood trim and marble; large entries.