Notes. The mock systems follow the base case with γ_ext = 0.05, q = 0.8, 5400 s exposure time, and a very bright source (⟨m_QSO⟩ = 18.5 mag), but differ by the main lens mass profile which is a composite mass profile made of a baryonic component and a dark matter component, instead of being a SIE. The case represented in the first row is multipole free while the two bottom rows display a multipole a₄ = 0.01, either in discy or boxy orientation. The mock images are fitted with a power-law model (hence the bold characters to differentiate it from the base SIE modelling, following the conventions of Table 1) with either a free slope or a fixed slope. In the latter case, the fixed value is the value retrieved in the no multipole case for each configuration.