Parshanut su Numeri 4:53

To perform service. The Torah does not use the language of melachah (creative activity prohibited on Shabbos) regarding the sons of Gershon and Merori, as it does here, because they carried using wagons and animals, which is only mechameir (guiding an animal), and not a melachah. On Shabbos it would only be liable for lashes. The sons of Kehos, however, carried on their shoulders, which is a hotza’ah (carrying), an important and complete melachah.