Is it good practice or required to limit the topics of public comment to agenda items only?
There is no legal requirement to allow public comment at all, or to limit it either to agenda items only, or to non-agenda items. Your policy BDDH may require it be held and spell out the specifics for your district. Many boards limit comment to agenda items but I have also observed boards who limit it to non-agenda items. The second case does not really make sense to me - I would think you would want to allow comment on agenda items. Some boards hold public comment twice, once at the beginning of the agenda and once at the end. One time is for agenda items, and the other for non-agenda items.
Most board just call the agenda item ‘public comment’, hold it at the beginning of the meeting, and take comments on both agenda and non-agenda items - even IF their policy says that they limit it to only agenda items. Someone who really wants to can relate almost anything they wish to an agenda item. Creativity knows no bounds.
In terms of 'good practice':
It is good practice to allow public comment, but to limit the amount of time for any one speaker, and to limit the total amount of time spent on public comment. This allows the board to proceed with their planned agenda and necessary work. Typical limits are 3 minutes per speaker, 30 minutes total for the session.
Public comment should be just that, and NOT a discussion with the board. The board should not answer questions on the spot unless they are very simple obvious ones regarding well established agreed on facts, but should refer questions to the administration or board chair for answer at a later time. If an initial answer by the board chair causes a follow up question then the chair should definitely refer the speaker to an administrator for further answers - the answer wasn't quite so obvious as seemed, or the speaker's intention is just to get the chair involved in a discussion.
The board should clearly state that they will not hear complaints against individuals in public, and that all complaints must go through the chain of command as spelled out in policy KL. This statement can be made in writing on the agenda, or by the board chair as an announcement at the beginning of the public comment session. Example:
Speakers may offer objective criticism of district operations and programs, but the Board will not hear complaints concerning individual district personnel. The chair will direct the visitor to the appropriate means for Board consideration and disposition of legitimate complaints involving individuals.
The board should limit the public's participation to the public comment section of the agenda. There should not be cross discussion between the board and the public during other agenda items.
All public comment should be directed respectfully to the board through the chair. Comments should not be directed at any one individual board member.
The board should clearly explain the 'rules' for public comment in their policy - usually policy BDDH, in writing on the agenda or a handout, and verbally by the chair at the start of the public comment section of the meeting. It is common to ask each speaker to identify themselves by name, or name and address, and many boards ask that speakers fill out a form with basic contact information before they are recognized to speak. All of that should be covered in policy BDDH.