What does "custom user permissions" refer to under the "Advanced License" features on the following page: https://cloud.google.com/terms/looker/legal/customers/licensing?hl=en
Platform Advanced - Looker Hosted includes one production Instance, 10 Standard User - Looker Hosted End Users, 2 Developer User - Looker Hosted End Users, upgrades, up to 100,000 query-based API calls per month, along with 1,000 administrative API calls per month, embedding, localization, custom user permissions, and custom license functionality.
Custom user permissions refers to the ability to create user permission sets other than the standard ones which are pre-built in the instance (admin, developer, standard, viewer). For example, creating "viewer who can't download".