These two authors recommend different approaches to document and part numbering. One of them speaks of structuring bills of materials such that multiple top levels contain unique parts and minimal additional levels containing parts common to all of the parents. So each level might or might not contain parts that can be assembled.  Has anyone had experience structuring bills this way and, if so, does it REQUIRE that one of the author's approaches to document and part numbering be used? Any related thoughts or experiences on this would be appreciated.