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For each added property you can decide to display it on portal’s edition or creation form, you can ou can manage this through portal’s structure facets configuration.
We recommend hiding the added properties in the default facet. Based on your needs, you can make them editable in the page facet (for portal creation) or in the page_update facet (for portal edition).
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You can find another example of property addition for segmentation purposes here : https://crossmedia.atlassian.net/wiki/spaces/WD/pages/3298787349/Portals+installation+guide#4.3%2F-Segmentation-of-portals