Just like an array with a computed number of occurrences, I believe a choice with direct dispatch should have no PoU.
But the spec has this phrase "An xs:choice is always a point of
uncertainty. It is resolved sequentially, or by direct dispatch."
Which suggests there is a role for asserts/discriminators in resolving a choice by direct dispatch even though there shouldn't be.
I think we should clarify this to "An xs:choice either is a point of uncertainty, or uses direct dispatch."