The mass promotion feature will check the rarity of your characters, as well as their levels, to determine how to promote them. Higher level characters will be promoted, and the lower level characters will be consumed (used as components).
By going to the Promotion view and tapping on the settings-button, you’ll be able to select how you’d prefer to utilize the mass promotion feature. You can choose to promote all possible characters, to promote specific rarity characters, or to do each completely manually. Whatever fits your style and strategy, you decide!
For more information on how promotion works in general, please tap on the info-button in the Promotion view in-game. You can also read "What is Character Promotion and how does it work?" FAQ for more information.