Clarification on "Context-specific usage"

Currently, at the end of http://getbootstrap.com/css/#buttons-disabled there's the callout:

Context-specific usage While button classes can be used on <a> and <button> elements, only <button> elements are supported within our nav and navbar components.

I'm not quite clear here what exactly this refers to...is it related just to the .disabled class, as that's the heading/section this callout is under? Or does it also apply to .active, in which case this needs to be "one level up" in terms of where it's located in the doc's hierarchy?

Further, does the second part of the sentence actually mean "... , they are only supported for <button> elements when used within a nav or navbar component." ?

/cc @cvrebert