trigger-scope gives authors the ability to limit the names of animation triggers declared by trigger-instantiating properties. Trigger-Instantiating properties, such as timeline-trigger, declare names which can be referenced by the animation-trigger property in order to attach animations to triggers. However, these names are global by default (similar to anchor-name) and it is often useful for author to limit the visibility of the names so as to isolate animation-to-trigger interactions.
Similar to anchor-scope[1], trigger-instantiating properties need a mechanism for limiting the visibility of the names declared by the trigger-instantiating property. [1] https://drafts.csswg.org/css-anchor-position-1/#anchor-scope
Explainers: https://github.com/explainers-by-googlers/scroll-triggered-animations/blob/main/TRIGGER_SCOPE.md