According to Microsoft:

Focused Inbox helps you focus on the emails that matter most to you. It separates your inbox into two tabs—Focused and Other. Your most important emails are on the Focused tab while the rest remain easily accessible—but out of the way—on the Other tab. You’ll be informed about email flowing to Other, and you can switch between tabs at any time to take a quick look.

However, if you find the concept of Focused Invoice annoying or counter-productive, it’s fairly easy to disable it.

In the Outlook desktop app

1. Click View
2. Clicked Show Focused Inbox to deselect it

In Outlook.com

1. Sign in to Outlook.com with your work account
2. Click Settings, the gear icon on the top-right corner of the page.
3. Click Display settings
4. Click Focused Inbox
5. Select Don’t sort messages
6. Click OK
7. Click OK again, then close Settings