Condensation (C) domains are the key component linking different monomers together, typically forming peptide bonds and occasionally ester bonds, during the nonribosomal peptide synthetase (NRPS). While A domains have been well characterised due to their role in selectivity of the monomers and functioning as a gate keeper in the NRPS biosynthesis, C domains have been a subject of debate as they have not demonstrated signs of “A-domain like” side chain selectivity of its acceptor side. Here, we report our biochemical and structural characterisation of the selectivity of the fuscachelin C3-domain showing that it is not broadly flexible for monomers at the acceptor site, suggesting the need to consider C-domain mutation regarding future NRPS engineering.