Primary key : The unique primary key should point to either the natural key, or the surrogate key (RecId).
Clustered index : is the physical order in which records are stored in the table. Say you have a clustered index on a text field called “Name”, and you insert a record with a name that starts with a letter D, the record will physically be inserted between the C and the E record. Clustered indexes do not have to be unique. ( ref : dynamicsuser )
Replacement key : indicate which fields are displayed on the UI, regardless of the table’s primary key ( ref : axinternals )