7.2.2 Folder Children POST needs clarification

In the create section, about content streams. Says: "A content stream MUST be specified if required". Then later, on listing the possible ways to supply a content stream, says it "MAY be provided ... At a later time". How is a server to know that a required stream will be provided at a later time.

If the stream MUST be supplied, and one of the allowed ways to supply it is "at a later time", then how does the repository know when/if the constraint (stream required) is satisfied? I'm just reading the words that are in the spec, and trying to figure out how to implement that. If something else is intended, then the words should say what is intended.