In section 4.4, some prose is devoted to explaining to effect of capturing the closure, notably:

> [...] are defined to retain the XPath static and dynamic context at the point 
> of invocation as part of the closure of the resulting function item, and to 
> use this preserved context when a dynamic function call is subsequently made 
> using the function item. 

This text says "at the point of invocation". But it is not an invocation, that occurs later, which is explained in the second part of the sentence.

What I think it means to say: "at the point where the function item is declared (for doc#x or doc(?)) or looked up (for function-lookup)"

I don't think the intend of this text is unclear, but I needed to read it several times to understand that "invocation" does *not* mean invocation of the function in question.