The description of the attributes in http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-att.coordinated.html has some problems:
1) Surely the @ul, @lr attributes are mutually exclusive of the @points attribute? That should be stated at least, and perhaps even a schematron rule added to enforce it.
2) There are no examples of the use of @start (at least none surfaced in the "show all examples" of <zone>. Presumably it points at an element like <lb> (assuming the zone describes a line of text)?
3) The wording of @points is misleading: there's nothing stopping you from describing a rectangle using @points. In addition, the wording reads to me as though it's meant to be used in conjunction with the @ul, @lr attributes to describe a ploygon contained by that box, but the examples don't give that impression (and that wouldn't make a great deal of sense).
Yes, @points should be mutually exclusive of the other attributes and we should add a schematron rule to enforce that.
An example of the use of @start is needed. Can you suggest one?
The situation is possibly a little complicated by recent changes to support transcription within <zone>, which has involved some other revision of the text.
We agree examples are needed. The @ul @ur attributes are needed at some level to define the coordinate space used by nested elements, and there are cases where its useful to supply both these and @points on the same element.
#1: there's no consensus on this, so it seems unnecessary to add this restriction; if the user wants to specify both a rectangular bounding box and a more nuanced polygon, I see no reason why they shouldn't.
#2: Chapter 11 now contains an example of @start. Perhaps it's arguable that more are needed, but the chapter is already inordinately long, unfortunately.
#3: The wording of @points has now been changed (revision 9857), as have various similar references in the draft of Chapter 11.