Single Hadron Multiplicities in SIDIS @ COMPASS

Single hadron multiplicities in Semi-Inclusive Deep Inelastic Scattering (SIDIS) provide an important input for the understanding of the hadronisation process and its description in terms of Fragmentation Functions (FFs). COMPASS collaboration has undertaken a programme of measurements of these observables. I first give an overview of the results. And I then focus on the $K^-/K^+$ multiplicity ratio at large value of the energy fraction carried by the kaons, which results challenge the conventional picture of a factorisation between FFs and Parton Distribution Functions (PDFs).