Engineered Hardwood. Just went down that road and found this....
: National Wood Flooring Association, a defect performance standard.
Part III
Subfloor Flatness and Integrity
A. Wood subfloors must be flat, clean, dry, structurally sound, free of squeaks and free of protruding fasteners.
1. For installations using mechanical fasteners of 1½” and longer, the subfloor should be flat to within ¼” in 10 feet or 3/16” in 6 feet radius.
2. For glue-down installations and installations using mechanical fasteners of less than 1½”, the subfloor should be flat to within 3/16” in 10 feet or 1/8” in 6 feet radius.
B. If peaks or valleys in the subfloor exceed the tolerances specified above, sand down the high spots and fill the low spots with a leveling compound or other material approved for use under wood flooring. However, it is the builder’s or general contractor’s responsibility to provide the wood-flooring contractor with a subfloor that is within the tolerances listed above.