You can bulk apply a custom attribute to a selected audience.
A custom attribute allows you to tag/group your audience in specific ways.
Once you've created your custom attribute, select the audience to which you would like to bulk apply that attribute. You can do so by clicking on the Audience tool > Segments or Lists.
Click on the "Take Action" button next to the segment or list to apply your attribute.
Click on the "Apply Attribute" option as you scroll down.
Select the custom attribute you would like to bulk apply to this audience through the "Attribute" drop-down.
Click on "Apply" after selecting your custom attribute and you're all set!