It only takes a few inches to wind up with a substantial drainage problem.
If it's in northing and easting, the baselines are to the South and West of all coordinates covered by the projection, plus an offset. So it might start at 1000000 instead of zero, to ensure format consistency and to catch blunders.
So it's about 2ppm, multiplied by the number of feet between you and the Southern or Western most edge of your state (if single projection state) plus 10^5 or 10^6 or similar.
It's designed to make an error large enough to notice.
Vertical however is rarely a problem, as it's all based off local benchmarks.
From the link below you can see the noaa benchmark is using both elevation in feet above sea level and in meters from the center of the earth.
Usually there's not even a reason to use H directly anymore, since elevations should reference a local benchmark.
GPS has changed things, but surveyors are still obscenely practical when it comes to procedures for eliminating systematic error.
