This article shares some typical use cases you can support, as well as an example flow that supports each use case. You're certainly not limited to these use cases, but they can be a helpful jumping off point as you plan your implementation.
If your customer should have their own Wix account ID and user ID:
Call Create Account on behalf of your customer, with the relevant email and SSO identities.
The account that is created for the customer is a child account under your company's parent account, sometimes referred to as a managed account.
Note that a new user ID will be created even if the customer already has their own Wix account with the same email address.
The customer's site will inherit any custom branding from your company's account, as well as some configuration, based on your company's contract with Wix.
If you create accounts for your customers, you can retrieve a list of all account IDs created under your account by calling List Child Accounts.
If you maintain management access to your customers' accounts, Wix recommends mapping these account IDs to your customers, so you can easily make account-level API calls that require passing an account ID in the header.