Heliocentric velocity relative to M31's systemic velocity (v _M31= −300 km s⁻¹) vs. projected M31-centric radius in the NE shelf fields for stars with successful radial velocity measurements, including both M31 RGB stars (colored points) and MW foreground stars (gray crosses; Section 3.1; left panel) and solely M31 RGB stars color-coded by their kinematically based probability of belonging to substructure (Section 3.2.2; right panel). Velocity errors associated with individual data points are not shown for the sake of clarity (median 5.3 km s⁻¹). The “wedge” pattern visible in this space for M31 RGB stars is characteristic of a tidal shell formed in a radial merger (Section 3.4.1). The right panel also shows the observed upper envelope of the wedge (white open circles; Equation (4)) and its best-fit model (black dotted line, with 1σ confidence interval as gray shaded region; Merrifield & Kuijken 1998) used to place constraints on M31's gravitational potential (Section 3.4.2).