Index AmInsert Parameter Confused?

[jacktby jacktby]

typedef bool (*aminsert_function) (Relation indexRelation,
                                                                   Datum 
*values,
                                                                   bool *isnull,
                                                                   ItemPointer 
heap_tid,
                                                                   Relation 
heapRelation,
                                                                   
IndexUniqueCheck checkUnique,
                                                                   bool 
indexUnchanged,
                                                                   struct 
IndexInfo *indexInfo);

Why is there a heap_tid, We haven’t inserted the value, so where does it from ?