As you all know that Dynamics Ax uses recid(Record Id)to uniquelly identify a record in the database.
By default system uses Recid as primary index,so if query optimizer didn't find any index to use then it will use RecId.
Dyamics Ax uses SystemSequence class(AOT->SystemDocumentation->Classes) to generate recId for table.
Using SystemSequence class you can reserve RECID's for your table.
Ax Kernel uses SystemSequences Table to generate the Recid for table as shown below
And sometime many of us tries to generate rec id in SQL, even this is possible just by a sample query described here: