Repeated choice with some but not all mandatory (a co-constraint use case)
Allow the specification of groups of elements in which the order of elements does not matter, certain of the elements are mandatory, others optional, and the maximum number of occurrences of a particular element is not limited to 1.
source: Regis Piccand 
Other use cases: Co-constraint Use Cases
I'm trying to have an unordered list of elements that can be "mandatory OR optional" AND repeatable (i.e. elements are unordered and minOccurs and maxOccurs must be specified for each element).
Using "All", maxOccurs is bound to 1 (optional but not repeatable). Using "sequence", elements are ordered (optional and repeatable). Using "choice", elements are optional (repeatable but not mandatory).
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This is a classic formulation of the use case for removing the constraint that says elements in an all group must have maxOccurs = 1. It can be done with co-constraints, but the stronger all group seems preferable.