On the face of it, there's a simple explanation of what a single-tenant or a multi-tenant application is. We can say that single-tenant apps are only available in the tenant they were registered. On the other hand, multi-tenant apps are available to users in both your home tenant and in other tenants:
- Single-tenantapplications are primarily used if you want to isolate your application from everyone and provide access only to your internal users
- Multi-tenantapplications are used when you want to provide the app to your internal staff, guest users, and Microsoft personal account users in a collaboration scenario, for example
We highly recommend reviewing https://docs.microsoft.com/en-us/azure/active-directory/develop/howto-convert-app-to-be-multi-tenant to find the differences between the endpoints that are used in both scenarios.