It is always a heading, introducing the subject matter that is to come. From the starting point of the title, the narrative traces the genealogy or the records or the particulars involved.
They further note that,
The subject matter of each תּוֹלְדֹת (tolÿdot, “this is the account of”) section of the book traces a decline or a deterioration through to the next beginning point, and each is thereby a microcosm of the book which begins with divine blessing in the garden, and ends with a coffin in Egypt.
Yet each decline is not without some spark of hope in divine intervention - thank God.
