The definition of Hesperia Planum used by the 2019 JGR paper corresponds to the USGS-defined boundaries of Hesperia Planum.
Hesperia Planum is a ridged plain covering approximately 2 million sq. km of the Martian surface. Hesperia Planum has traditionally been interpreted a flood lava plain, based on its similar appearance to the Lunar mare basalt plains (e.g. Scott and Carr, 1978; Greeley and Spudis, 1987). However, work by Tracy Gregg has suggested that Hesperia Planum may be composed primarily of pyroclastic materials erupted by the volcanic centers at Tyrrhenus Mons and Hadriacus Mons (e.g. Gregg and Farley, 2006), primarily due to a relative lack of observable lava flows compared to other volcanic centers.