When you enable filterable gallery for videos, this option appears. On page load, you might have observed that the default 「All」 tab is displayed. You can change this tab with a Default Tab on Page Load option.
The setting can be found under the Video > Filterable Tabs > Default Tab on Page Load.
Choose option Custom and enter a category name that you want to set as a default tab. That means the category name that you will enter will display as the active tab on the page load.
Note: Make sure you enter the exact same name for the category set for the video. Refer How to Set Categories for videos?
Here is an example of how it works –
Consider some of the videos are assigned a category Lifestyle.And you wish to display the Lifestyle category tab active on page load.Choose option as 『Custom』. Then enter a Default Tab as Lifestyle ( make sure the name is exactly the same as the category name set for the videos ).Now on the page load, Lifestyle tab will be displayed as an active tab, and videos that are assigned a category Lifestyle will display under it.
Note: If you want to display 「All」 tab as a Default Tab, leave the option to FIRST.
Related documents –
How to Design Filterable Video Gallery?