How to Delete a Dimension from a Position
Login to Ad Manager. Scroll down to the bottom of the page and click on:
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Click Zone Configuration on the next page:
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Make sure you're in the right web property. If you're not, just click on the drop-down menu and select the property you wish to work on:
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Go to the position and click the pencil icon to reveal the dimensions assigned to it:
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To delete a dimension, click the trash can icon next to it:
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Changes to dimensions are auto-saved.