Secondary Menu – Under Main Menu? Or Per Page Usage?

Hello, Can you please explain/send me to documentation on how the secondary menu works. When I enable it, it appears next to my main menu. (see attached)

Also, is there a way to change to a secondary menu per page?

For example, when a user is in my blog part of the site, can I show a blog category only menu across the top? Or if they are in my shopping page, can I show a shopping category-specific menu either under the main or in replace of the main?

I had asked about it https://archived.seventhqueen.com/forums/topic/visual-composer-does-not-show-text-in-text-box-backend-or-front-end-editor but think it got lost in the rubble of the error messages.

Thank you. 🙂