Thank you for your post.
If "Reference" is open, then the relationship between Header and Detail with "Delete related recordis in this table when a record is deleted in the other table" is active, so if a record is deleted from Header, it will delete the related record from Detail. With "Reference" not open, the "Delete related records" is not active because Main does not relate to Reference.
Thank you for your reply, however it is completely different than my current understanding of table occurrences and base tables.
As an example, within the same file I can create table occurrences A and B without "Delete related records", and create another set of occurrences A' and B' with "Delete related records", and the delete related records will apply when I delete records from layouts using A' or A. I would think it would only apply when deleting records from A' because only the A'B' relation is checked. Why does a check not appear in the AB relation when I set it in the A'B' relation? I haven't tested yet as of writing this, but do the "Allow creation" and "Sort" also apply to all occurrences (and open files) as well?
I was under the impression that the settings of a relationship only applied to that relationship. In what cases is having the settings apply to all open files of benefit? Is it a side effect of the internals or FileMaker? I guess these are the questions I'm trying to answer.
With just "Main" open, the Relationships graph shows "Header" and "Detail" linked together without the option to delete records. There is no table occurrence displayed for "Reference".
When "Reference" is opened, the Relationships graph shows "Header" and "Detail" linked together with the option to delete records in "Detail". Therefore, when you delete a record in "Header", although "Main" doesn't show the option to delete records, "Reference" does show the option, so the recod is deleted in "Detail".
Without "Reference" open, there is no reference (no pun intended) in Main to access "Reference" and delete the record, so only the record in "Header" is removed, and not in "Detail".