A primary key is one type of key. The other type of key is an alternate key. There is a maximum of one primary key per table, whereas a table can have several alternate keys. The primary key is usually the type of key that other tables, called child tables, refer to when a foreign key field in those other tables need a relational identifier.
Starting in Microsoft Dynamics AX 2012 the primary key for every new table is always enforced by an index that has exactly one field. The one field is usually an incremented number or a completely meaningless number that is generated by the system. For new tables the default is a primary key based on the RecId field. This is represented as the surrogate key in the user interface.
The following describes the PrimaryIndex property and other that are related to keys.
The drop-down list contains the surrogate key plus every index on the table that has its AlternateKey property set to Yes.