Illustration of the gain in information on the long-term evolution of X-ray sources, obtained thanks to the cross-matching & upper-limits. Top panel: distribution of the number of catalogs involved in each multi-catalog source. The majority of the sources only have data for one catalog, but for the remaining 15% at least two catalogs are involved. Despite using 7 catalogs, no source was detected in all of them (mostly due to the very constraining sky coverage of the eFEDS catalogs). Bottom panel: density distribution of the number of data points per source, before the cross-match in light blue, after the match in blue, and after taking into account upper limits in dark blue. Both the cross-match and the use of upper limits allows us to increase the number of data points per source, namely, skew this density distribution to the right.