  Recent observations have revealed the presence of small fibres or
sub-filaments within larger filaments. We present a numerical fragmentation
study of fibrous filaments investigating the link between cores and
sub-filaments using hydrodynamical simulations performed with the moving-mesh
code Arepo. Our study suggests that cores form in two environments: (i) as
isolated cores, or small chains of cores, on a single sub-filament, or (ii) as
an ensemble of cores located at the junction of sub-filaments. We term these
isolated and hub cores respectively. We show that these core populations are
statistically different from each other. Hub cores have a greater mean mass
than isolated cores, and the mass distribution of hub cores is significantly
wider than isolated cores. This fragmentation is reminiscent of parsec-scale
hub-filament systems, showing that the combination of turbulence and gravity
leads to similar fragmentation signatures on multiple scales, even within
filaments. Moreover, the fact that fragmentation proceeds through sub-filaments
suggests that there exists no characteristic fragmentation length-scale between
cores. This is in opposition to earlier theoretical works studying fibre-less
filaments which suggest a strong tendency towards the formation of
quasi-periodically spaced cores, but in better agreement with observations. We
also show tentative signs that global collapse of filaments preferentially form
cores at both filament ends, which are more massive and dense than other cores.
