The seventh aliyah is about the special duties of the Kehos family. After Aharon and his sons initially cover the most holy items of the sanctuary, the Kehos family are to further cover the items and, when the whole camp journeys, the Kehos family would actually carry these items. The holy items that they were thus responsible for included the Aron HaKodesh containing the luchos (the tablets with the Ten Commandments on them), the Menorah, the Golden alter, etc. The Kehos family are to start their work only after Aharon and his sons do the initial covering, so they (the Kehos family) would not die from touching these most holy things.