I'm a digital marketing expert specializing in brand research, detailed audience construction, intense demographic and psychographic targeting and providing REAL RESULTS! I have been in the digital advertising industry for a decade and have worked for large brands such as Microsoft, Johnson & Johnson, Adobe, Pulte Homes, American Apparel and Cleveland Clinic - with budgets as large as $500k/month. I have also worked with small businesses to create amazing results with starter budgets.
If you add your custom audience without any additional restrictions, only people in your custom audience will be exposed to your ads. If you add a demographic, or interests, then FB will require that a user a) is in your custom audience AND b) meets the additional qualifiers you have added.