Jamieson, Fausset and Brown on Deuteronomy 31:14-15

14-15. the Lord said unto Moses, ...call Joshua, and present yourselves in the tabernacle of the congregation--Joshua had been publicly designated to the office of commander by Moses [Nu 27:22, 23]; and God was pleased to confirm his appointment by the visible symbols of His presence and approval. As none but the priests were privileged to enter the sanctuary, it is probable that this significant manifestation of the cloudy pillar was made while the leaders stood at the door of the tabernacle.