Property bags are explained in my another article: Property Bags in SharePoint 2013 - Store, Retrieve Custom Configurations , In short: Property bag feature provides a convenient way to store and retrieve custom configurations, instead of having to store them in web.config files.
Usually property bag settings are managed programmatically with SharePoint object model code in C# ow with PowerShell. There is no user interface to view/add/edit/delete the property bags key-value, but SharePoint designer can be utilized to manage property bag settings in SharePoint at site level. Here is how:
- Open your SharePoint site collection with SharePoint Designer
- Click on the Site Options button from the top right hand corner of the ribbon
- You'll be presented with a window to manage site’s property bag under "Parameters" tab.
- From here, you can add, modify or delete properties.
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