This summary is not the rule. The rule text controls.
When someone dies and part of their property is covered by a will but part isn't, the estate is called "partially testate and partially intestate." This rule explains what needs to happen if the family wants to handle that kind of estate through independent administration, which is a simpler process that doesn't need constant court supervision.
Basically, everyone who has a legal stake in the estate has to agree. This includes the people who would inherit under the will and the people who would inherit under the state's inheritance laws for the part with no will. All of these people need to agree on two things: that they want independent administration, and who they want to serve as the independent administrator.
If even one person who's supposed to have a say doesn't agree, the estate can't use this simpler independent administration process. In that case, the estate would likely go through regular court-supervised administration instead. This rule is meant to make sure everyone with an interest in the estate has a voice before the family gets to skip the usual court oversight.