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Not every AF sensor can see a lighted target area at every focal length. It varies with flash unit (because of the focus assist pattern), the focal length of the lens and perhaps the model of the lens (because the FL determines where the AF sensors lie in relation to the image frame and hence to focus assist pattern). Therefore you will get a variety of results from different people who are trying it with different lenses and focal lengths.
With my D3s I can get the SB-900 focus assist to work regardless of whether the AF selection set is 51-point or restricted to 11-point. It works for single-point AF, dynamic area AF, and auto area AF. It also works regardless of whether the AF dynamic area is set to 9-point, or 21-point, or 51-point, or 3D 51-point.
It will not work with AF-C mode.
It will not work if the lens is set to MF mode.
It will not work if the AF has been set but subsequently locked.
It will not work if the flash is not ready (the battery hasn't yet delivered sufficient charge to the flash unit).
The D700 has, I think, a custom function to disable the use of the external flash focus assist because it also has an internal focus assist light. So this as well as the other conditions would prevent the SB-900 focus assist from working.
The fact that it works with all 11 points in 11-point selection mode is not guaranteed at every focal length. e.g. from 106mm to 135mm it is reduced to a vertical column and a horizontal row of AF sensors. So you might as well stick with 51-point selection if that is what you would normally prefer.
I have also found that the focus assist pattern can be hard to see through the D3s viewfinder but is quite evident when I do not look through the viewfinder. Maybe there is an IR or near-IR filter in the viewfinder.