Origin of the XXL-100-GC sample spectroscopic redshifts in different redshift slices. The available spectroscopic data are split into four categories: (i) the follow-up data obtained by the dedicated XXL ESO Large Program (LP, PI: C. Adami) using a combination of NTT/EFOSC2 and VLT/FORS2, together with previous data obtained by the XMM-LSS team with the same instruments; (ii) the XXL follow-up program based on the Anglo-Australian Telescope (AAT, PI: C. Lidman); (iii) the XXL William Herschel Telescope (WHT) follow-up program (PI: B. Poggianti); and (iv) redshifts obtained through partner spectroscopic surveys, including GAMA (Driver et al. 2011), VIPERS (Garilli et al. 2014) and the VVDS (Le Fèvre et al. 2013). The observations and data reduction for the redshifts obtained at the WHT and AAT are respectively the subject of XXL Paper XII and Paper XIV. For each category, the plot shows the proportion of clusters with galaxy redshifts coming from the corresponding project.