Im carefully studying Deep Inelastic Scattering and I have the doubt if valence quarks are real physical entities or if they are just a way we can think hadrons are built. I understand that inside hadron we have the Parton Distribution Functions that say that there are an infinite number of quarks, antiquarks and gluons. In many lectures (https://gsalam.web.cern.ch/gsalam/repository/talks/2009-Bautzen-lecture2.pdf, slide 27) I can see that valence quarks are defined as the integral of the quark minus antiquark PDF distributions. This makes me wonder if the valence quarks are just a way to imagine the composition of hadrons, in the same way the Bohr model hepls to visualize the atom. Thanks for your help!