The documentation on CellGroupingRules has a warning that it is not fully integrated.  However, because it is used in the standard supplied StyleSheets, I am guessing that it may be around for a while.

In any case, the documentation for CellGroupingRules is sparse. Can anyone point me to something with a little more detail?
I am trying to back out what CellGroupingRules should do by looking at various style sheets---but I am hopeful that someone might have a pointer.

In my case, I'm building a private style sheet from scratch.  I'm not using the Default styles, so my private style sheet starts like this:
I'd like to control automatic grouping.  As a familiar example, here might be a set of Styles for a private stylesheet for a book. (This is not what I am doing, but  hopefully serves to illustrate)

I've tried doing this by using manual grouping in the style sheet editor, this doesn't seem to work.

Style Book always starts a new cell group
Style Chapter is subgrouped with its particular Book
Style Section is subgrouped with its particular Chapter

Any FigureTitle is subgrouped to whatever is immediately above (*having the effect of closing the figures and captions and retaining the title*)
Any Figure is subgrouped to a FigureTitle
Any FigureCaption is subgrouped to  a Figure

Any Text is subgrouped to whatever is immediately above.

Any  ReferencesList is subgrouped to a Chapter, but is not subgrouped to Section

Any IndexList is subgrouped to a Book, but nothing else.