How to treat Apple SMS as a primary card or a secondary card?

Regarding the question of whether dual-card SMS is sent to the main card or the secondary card, in fact, Apple has long distinguished it, but it is not intuitive enough.

There is an arrow to the right of the name or number at the top of the middle. Click to point the arrow down, and you will see an information option. Click enter, and you will see a prompt: You are using "major number" or "minor number" for this conversation.

It's really not intuitive for Apple to set up this kind of dialogue between primary and secondary numbers here. I hope it can be set up when the system is updated next time, so you can see it without clicking.