For a constitutional repeating unit of a dendron, the direction from the free valence to the end-groups.
Source:
PAC, 2019, 91, 523. 'Nomenclature and terminology for dendrimers with regular dendrons and for hyperbranched polymers (IUPAC Recommendations 2017)' on page 529 (https://doi.org/10.1515/pac-2016-1217)