[beerandt]

There's a ppm error there as well, but that's a usually a different problem. If different units were used, it would be obvious when checking in on the benchmark.

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.