Hello,
Each alternate key relies on a unique index for its implementation and enforcement. However, a unique index alone does not make an alternate key. The AlternateKey property must be set to Yes to make a unique index be an alternate key, so that other tables can create foreign key relations that reference this key, as an alternative to referencing the primary key.
If you set the AlternateKey property of an index set to Yes, it can be choosen as a replacement key.
A replacement key is an alternate key that the system can display on forms instead of a meaningless numeric primary key value. Each table can have a maximum of one replacement key.
The drop-down list contains every index that has its AlternateKey property set to Yes.
Please refer this topic for more details: docs.microsoft.com/.../table-keys-surrogate-alternate-replacement-primary-and-foreign
