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The Work Item Selector field determines which Work Items are considered during this Gate evaluation. There are 3 selector options when it comes to Work Items in the Release and Pipeline executionavailable:
Release Work Items - All Work Items ever associated to the Release (these show up under the Work Items section on the Release Definition tab).
Snapshot Work Items - These are the Work Items that were associated during build of the current Release Snapshot. These are generally a subset of the Release Work Items.
Stage Work Items - Stage Work Items are all Work Items that will deploy in the upcoming Stage steps.
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