Completeness of Gaia relative to the DECaPS survey, which is taken as ‘ground truth’, in four magnitude ranges, computed in 3000 distinct patches across the DECaPS footprint. This completeness is shown as a function of Gaia source density (left) or M₁₀ (right): Gaia source density is a poor predictor of completeness, while the M₁₀ parameter – the median magnitude of catalogued sources with astrometric_matched_transits≤10 in a surrounding patch of the sky – is an excellent completeness predictor. M₁₀ combines the impact of source density and scanning law, as demonstrated e.g. in Fig. 2.