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It’s possible, for example, to have a box with a subset of initiatives or epics and their children.
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You will typically create the Extended search filter for the board and reuse it for the box.
The process of creating the filter from scratch looks like this:
Create a filter in Extended search
Wait for filters to be synced in BigPicture BigPicture to sync the filters list (see the note below)
Use the filter in BigPicture’s box configuration Task → Scope Definition → Automatic Rules
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