Is Cognito pool user sub attribute globally unique?

I tried in past with common sub and it didn't allow me to do so , i can say sub is a globally unique identifier, across pools . Basically when creating a user , Cognito will assign a generated unique IDs (the sub attribute). This attribute cannot be changed and in case you import users from another pool/backup it will change. Just on side-note sub risks data loss and can make it difficult to migrate or recover a userpool. Since sub is globally unique, any restored user data will have new sub values. It will mean re-keying your app database with the new sub keys, whereas if you had used username you could restore your pool as expected.

Sub is globally unique in the whole Cognito User Pool ecosystem.