Typically, the chemical substance in question is first dissolved by adding 100% DMSO - to some extent for practical reasons, i.e. all the advantages of working with stock solutions. The main reason (to my mind) is, however, that compounds often do not dissolve easily in (aqueous) solutions of DMSO for kinetic reasons, but are perfectly happy when diluting with water afterwards.
For your downstream application, e.g. some biological or biochemical assay, the 100% solution is diluted with the assay buffer to e.g. 5% DMSO. The latter is almost exclusively volume per volume%.