It will be cross type with any 5.6 lenses and it will add an additional highly sensitive vertical line for f/2.8 and wider lenses.
Exactly. This is a new AF point configuration for Canon.
Previously, they had single lines (f/5.6, one otientation only), crosses (two lines with the same aperture threshold, like the outer points on xxD), two-line hybrid crosses (two lines with different aperture thresholds, like the center point on Rebel/xxxD and 5D/5DII, and all of the cross-type points on 1-series bodies before the 1D X), and dual crosses (four lines total - two pairs
of lines with different aperture thresholds, superimposed as an 'x' on a '+').
The 6D's center point is a hybrid cross with three lines - an f/5.6 cross ('+') with an additional f/2.8-threshold horizontal sensor line.
Your typical dSLR AF system doesn't focus precisely with lenses wider than f2.8.
False. The AF system is specified to a given level of precision - the f/5.6 lines are precise within one depth of focus at the lens' max aperture, the f/2.8 lines within one-third the depth of focus at the lens' max aperture. It doesn't matter whether the lens has an f/2.8 or an f/1.0 max aperture, the AF system will be precise to that degree relative to the max aperture
of the lens. Note that precision is specified, not accuracy. Precision ≠ accuracy.
The DoF at apertures wider than f/2.8, especially with reasonably close subjects, is so thin that any misalignment in the AF system will be exposed - that's the accuracy part. That's why we have AFMA - to correct for the inaccuracy introduced by misalignment of the AF sensor with the image sensor. A properly microadjusted lens will be accurate, and precise within the limits stated above, i.e. the obtained focus will be distributed around the 'true' focus in a manner bounded within 1/3 or 1 depth of focus (probably not hard boundaries, but likely about 3σ, which is a standard tolerance in most processes).
focusing a lens wider than f/2.8 through the viewfinder with a stock focus screen is not
accurate or precise, because the stock screen does not show the true DoF of a lens faster than ~f/2.8. The AF system will do a lot better (if properly adjusted).