The formation of stars is a “social” process. It leads to the assembly of young stars at various length-scales into structures of diverse degrees of self-binding, from gravitationally-bound star clusters to unbound stellar associations, and beyond (e.g., Elmegreen 2000). These different young stellar systems are, however, not independent to each other. In nearby star-forming regions compact clusters appear at few 1-pc scales as the nested centers of star formation within loose stellar aggregations of few 10-pc sizes (e.g., Schmeja et al. 2009). Resolved populations across whole galaxies show that these structures are themselves components of larger stellar complexes with typical sizes of few 100-pc, and this structural behavior seems to extend to kpc scales in galactic super-structures and spiral arms (e.g., Gouliermis et al. 2015). From the stellar clustering point-of-view, star formation behaves as a scale-free process, expressed by power-laws in size distributions and correlation functions (e.g., Elmegreen et al. 2014). The origins of the self-similar stellar structural morphology, the scales where it changes behavior, and how these scales are determined, are fundamental questions to our understanding of star formation.