1. Introduction  
2. Selection of the $\pmb{\chi}$-ray cluster sample  
3. Spectroscopic redshifts  
3.1. XXL extended sources spectroscopic follow-up campaigns  
3.2. Redshift reliability and precision  
3.3. Cluster spectroscopic confirmation  
4. The cluster catalogue  
4.1. Sample description  
4.2. X-ray direct measurements  
4.3. Cluster parameters from scaling relations  
5. Updated cluster statistics  
5.1. Redshift distribution and spectroscopic redshift sampling  
5.2. X-ray luminosities and fluxes  
5.3. Luminosity-temperature relation of the C1 sample  
5.4. X-ray luminosity function  
6. Witnessing the evolution of massive structures: from super-clusters to fully collapsed fossil groups  
6.1. Super-clusters  
6.2. Merging process: the peculiar case of the XLSSC 110 system  
6.3. Fossil groups  
7. Conclusion