A data context for the instantiated item.

This works with features like MatchKey, behaving like an Each with a single item in it.

For example, you may have part of the UI depend on the type of data being viewed:

<Instance Item="{card}" MatchKey="type">
    <NumericCard ux:Template="number"/>
    <FaceCard ux:Template="face"/>
    <JokerCard ux:Template="joker"/>
</Instance>