A stud mounted rocker arm needs to have a means of keeping the rocker aligned with the valve tip. The two common ways are guiding the rocker from the pushrod side (guide plate or slot in the cylinder head) or by capturing the valve tip (self-aligning) in the rocker arm.
OR It is important that this is only done by either one of the two methods. In other words, the rocker must be held in alignment with the valve tip, but it cannot be guided from both the pushrod and the valve side at the same time. If your cylinder head has a slot machined into it or a pushrod guide plate, non self-aligning rocker arms should be used. If you don’t have either of these then look at your stock rockers, if you see an indentation in the underside of the rocker that would capture the valve tip, then you would use self-aligning rocker arms.
A word of caution: some aluminum cylinder heads from GM had a ‘guide plate’ on them and still used self-aligning rocker arms. This ‘guide plate’ had wide pushrod slots in it and was used for engine assembly purposes at the factory only. This type of ‘guide plate’ is not for holding the rocker arms in alignment, so these heads would use self-aligning rockers.