Example case of blending, where a bright AGN drives the detection and the presence of a faint extended source favours its classification as extended. This figure is a cut-out of a simulated count image, smoothed with a Gaussian. The red 2- and 5-arcmin circles are centred on a faint simulated cluster (filled triangle symbol) with flux ∼5 × 10⁻¹⁴ erg s⁻¹ cm⁻². Detected sources appear with blue squares. A detection close to the cluster centre is classified as extended (magenta circle). It also coincides with a bright simulated AGN (orange “x”). The position-based matching algorithm does consider the simulated cluster as selected (I_main = 1), contrary to the photon-based matching procedure. Two other faint simulated clusters are shown with open triangles, they have no impact on the detection.