Primary Index
A primary key is one or more columns that uniquely identify one record in a table from all the other records. A primary index is a unique index for a table that defines the primary key for that table. To set an index as a unique index, set the index property Allow Duplicates to No.
To set the primary index on a table, open the property sheet for the table. The PrimaryIndex property allows any unique index with a key that is mandatory and cannot be edited.
Surrogate Key
When a primary index is not specified, Microsoft Dynamics AX 2012 uses a Surrogate Key as the primary index. This key is the RecId field and, if the table is saved per company, the DataAreaId. The surrogate key is used on many relations between tables.