document.browsingTopics() vs fetch call with browsingTopics param

[relevance-api-topics]

Hi Topics API experts,

Hope to ask a question to confirm 1) document.browsingTopics() vs 2) fetch call with browsingTopics param 's functionality comparison:

• For getting topics:
  Both 1) document.browsingTopics() and 2) fetch call with browsingTopics param get topics based on that current web-page host name is the observer.

• For marking observer:
  Calling 1) document.browsingTopics() does not mark any observer/observation, only 2) fetch call with browsingTopics param can mark observation. And if from websiteA there is a fetch call towards websiteB, only websiteB is marked as observer based on websiteA's inferred topics, websiteA is not marked as observer for the call.

Question: if Topics API is designed as a tool supporting cookie deprecation, why when doing a fetch call from websiteA to websiteB, websiteB's observing topics cannot be filled? (for cookie case: websiteB put a cookie on websiteA, and when websiteA call websiteB the cookie websiteB setup before can be sent together)

Just to confirm above is by-design, or if we missed anything, sorry if above question is not clear enough, thanks!