The UDMF format allows any number of custom properties on every element in the map. With the custom fields editor, you can edit the known properties and add custom ones yourself. The game configuration has known properties that are supported by the sourceport and are automatically shown in the list. Grayed items are known, but have not been set (and thus have their default value). To add your own custom properties, you can click on the text Click to add custom field and type the name for the new property. Then you can change the type and set it's value. To remove a custom property, just select it and press Delete. Known properties cannot be deleted, pressing Delete with a known property selected will just reset it back to it's default value.