If this is “big-endian”, then what is this?
That's an interesting question. I would argue that the answer to your question lies in implicit assumptions that you (are willing to) make and expecting others to make.

In the ISO 8601 example, the significance is well defined, as 1 year = 365 days, we could say that a year is more significant than a day. In the last data structure example, what do you mean with significant? There is no clear definition there. Without framing what context you are referring to (such as level of abstraction or chronological order), interpretations are bound to differ to the people you are communicating with (and that defies the purpose of using the communication strategy, doesn't it?). 

Instead of compressing the idea to a single concept, you could consider to be as explicit and unambiguous in the relation between the data structures as possible.