What is the difference between using a Microsoft Account vs Work account?
A Microsoft Account (MSA) is a personal email account owned by an individual to access Microsoft services when an organization is not using a managed tenant for Microsoft 365 or Azure Active Directory. A work account is one owned by the organization that issues it and is used to access services on Microsoft 365 or Azure Active Directory tenants. For complete information about the differences between MSAs and work accounts, see 'Learn about the differences in using MSAs (Microsoft Accounts) vs. Work Accounts (Azure Active Directory or Microsoft 365) and how this may impact your organization'.
What does it mean when my subscribers add alternate identities?
Adding an alternate account allows your developers to access some of their Visual Studio subscription benefits (VS IDE, Azure DevOps and Azure) with a different identity than that which the subscription is assigned to. This doesn't provide a copy of subscription to the other account; it only provides the ability to access the two benefits with the alternate account. Read more about Visual Studio Subscription identities.