Not necessarily to start up the shooting again, but my experience is that the traditional definition of "center hold" is placing the sights in the center of the target.
Not placing bullet holes in the center of the target.
That's with the top of the front blade or post in the center of the target.
That's regardless & totally independent of where the bullet strikes.
The top of the front sight is in the middle of the black bull, the aiming point is in the middle of the black bull.
That has nothing to do with where the bullet actually strikes, which can be anywhere, including in the center of the bull at the aiming point, or someplace above, below, or to the side of the aiming point.
"Center hold" is a term relative to & based on ONLY the sight picture.
"Center hold" has nothing whatever to do with bullet impact location.
The traditional meaning of "6 o'clock hold" is also based entirely, exclusively, and solely on positioning the black bull (or dot) on top of the front sight.
Has no relationship to where the bullet actually strikes.
Center hold: Top of front sight positioned in center of black bull.
6 o'clock hold: Top of front sight right at lower edge of black bull.
Either hold can result in a bullet impact striking dead center in the black bull, if that's where you regulate your elevation adjustment to put it, but where the bullet hole is does not determine the terminology or description of your sight picture's "hold".
There is zero correlation between the two: point of aim and point of impact; in describing which method of "hold" you're using.
Denis