Custom properties are configured on a per-event basis and are limited to 255 characters. Any changes to a custom property take effect in less than 5 minutes.
Custom properties are triggered by a user’s interaction with the CSS selector in the defined event. As a result, they are available to use in analyzing the events on which they were defined, but will also be applied to other defined events if another event definition uses the same selector.
Note: Custom properties are not yet available for iOS.