Some themes or plugins use custom fields, starting with an underscore to store related information. End users will not see such custom fields on a list in Views GUI.
To use a hidden custom field in your Views, go to the Toolset → Settings page and click the Front-end Content tab. Click Select custom fields button in the Hidden custom fields section. In the dialog box that opens, select the desired field from the list and click Apply.
The hidden custom field you selected will now be available to use in Views, just like any other custom field. You can use it for ordering, filtering or display it in the Loop Output section. Typical usage scenarios for hidden fields include:
- Using hidden fields in loop output
- Using hidden fields in query filters
- Using hidden fields in ordering