ICOS observational data products comprise data collected at 100+ individual stations, each equipped with hundreds of sensors, so it is very important to record exactly where data values come from. Also the entire chain of processing steps - including calibrations, quality control, and gap filling - must be traceable. Some variables are calculated and/or aggregated from high-frequency observations using complex data processing, and to ensure reproducibility, information about which software versions and parameter sets were applied is crucial. Similar considerations are also applicable to ICOS “elaborated” data products.