Custom Apps for Business?

Ordinary dev program account. No additional costs to the dev.

You upload apps to the store that are then out of sight to the normal public, then sell behind the scenes to qualified buyers. See Apple's VPP/B2B programs...now morphed a bit into Apple Business Manager

The above responses by KMT are accurate. Custom Apps is part of the Apple Developer Program, and is simply a choice you make at time of publication under the Pricing & Availability section in App Store Connect. The review process is meant to be the same as the one you would go through for a public app. The reviewer will need to be able to fully use the app as intended, so please provide a demo mode or demo account for their use if they cannot create an account.

Your customer will need to install the app through ABM, and will need to have Custom Apps enabled there. Once you have entered their organization ID into App Store Connect when choosing private distribution, they should be able to see it in ABM for distribution to their users.

For more information, watch Session 304 from WWDC19 and Session 10667 available later this week

Will the new Session 10667 cover the "changes to Custom App Distribution" mentioned in the new license agreement? This seems to imply a free custom app could now be released to public App Store. We had an app that was pushed into the Custom Store due to limited audience judgement by Apple, and I am trying to figure out if we now can move it back to the public free store with this policy change.

Apps with a limited audience still need to be distributed through the Custom App Store privately. There has been no change to allow them on the public store.