A position based attribution model is an attribution model which gives conversion credit to digital marketing channels according to their places in the path to conversion.
More often, a bonus is given to the first and the last channel. The first for creating awareness and the last for closing the sale.
For instance, with the Doubleclick DFA default position based model, 40% credit is assigned to the first interaction, 20% credit is assigned to the middle interactions, and 40% credit is assigned to the last interaction.
See other attribution models.