Retention policies vs. measurements

I understand the basis behind retention policies but I do not understand what they are doing in the queries.

With the following: “some_database”.“autogen”.“data”

Does that mean that to access “data” I need to know what retention policy was used to write the data?

If “data” was made of records using two different policies, I wouldn’t be able to query them at once?

Does this also mean that “data” is in fact split by retention policy and might as well be totally different measurements?

Adding the retention policy is just another way to be more specific about your query. You can write a query without specifying the RP or add the RP to get data that is “split” by policy.