I'm a DIY on my second hardwood floor and the subfloor this time needs a lot more work. But I don't understand how to interpret the flatness spec:

"the subfloor should be flat to within minimum tolerance of 3/16” in 6’, or ¼” in 10’" pg. 58

I have a high spot at a joist crown and where two plywood seams meet. I know I need to sand this down, but how would one measure ¼” in 10’?

I'd assume you would place the 10' straight edge on the ground and measure the elevated end.
But you could change the elevated amount just by sliding the end towards the high spot (fulcrum) and get a different reading. Which reading conforms to the spec?

I found this video:
https://www.youtube.com/watch?v=w308Cn-M9tg

At one point the narrator says 8' is on the ground and it is a 1/2 elevated at the 10' mark. But by moving the grounded end towards the high spot that would increase the elevation quite a bit.

Thanks

You can measure with either a 6' or 10' straightedge. Start by setting the straightedge on the middle of the high spot. Of course the straightedge will rock. This affirms that this is a high area. You will need to plane or sand the high area until there is hardly any rocking on the ends. If you get the spot flat, there will not be any rocking. You will need to check both directions to make sure other areas on the joist are not high, which will rock at the ends. This is where the flatness recommendation will show the tolerance that you are working toward, with either a 6 or 10 foot straightedge. The floor need not be level, just flat. Bumps in the sub-floor will transfer to the finish floor and can cause stumbling or premature wear from high spots.
Check for low areas after the high spots are dealt with. I use thick roll roofing to build up low areas with layers like what a contour map looks like for different elevations because it is about an eighth inch thick. Use the straightedge to identify where the different layers of paper that will make the floor flat. You can be an eighth of an inch off and still be fine. Make a plan using one or two layers as a gauge under the straightedge to make a mark on the sub-floor where each layer needs to be built up. Then use a stapler or hammer tacker to keep each layer in place so it doesn't move around. I don't like to build up more than two layers cause I use 3/8th inch staples. Asphalt saturated felt will not deflect unless it melts so it is the easiest method to use as it will be ready to cover with AquaBar paper and lay the floor immediately. The paper will cover up any of the tiny gravel that may fall from the roll roofing, if you miss sweeping everything up.
When you get the whole floor flat you will be ready to lay the best floor.