Group area S as a function of the maximum probability within a group p_max. From left to right: Random fields (in the ACT footprint), groups associated with optically confirmed galaxy clusters from the test ACT DR5 data set and from the test PSZ2z subsample (in the ACT footprint), and SZcat directions (in the ACT footprint). With green colour we show a region where detected groups of pixels are classified as clusters: S > 20 and p_max > 0.8. We see that with this simple criterion in random fields almost all detected sources are classified as non-clusters (as expected), with only ∼10 per cent of groups falling into a ‘cluster region’. For ACT DR5 and PSZ2 optically confirmed clusters, we correctly classify ∼80 per cent of clusters.