Azure Managed Identities with System Assigned and User Assigned and AZURE_CLIENT_ID

I know that when you set AZURE_CLIENT_ID to the User Assigned client id then you do not need to set the ManagedIdentityClientId when calling DefaultAzureCredential(). However, if your resource has both a System Assigned managed identity and a User Assigned managed identity, when you call DefaultAzureCredential which managed identity will be used (System or User)?