Albert Barnes Commentary Numbers 9:15

"And on the day that the tabernacle was reared up the cloud covered the tabernacle, even the tent of the testimony: and at even it was upon the tabernacle as it were the appearance of fire, until morning." — Numbers 9:15 (ASV)

The cloud ... - The phenomenon first appeared at the Exodus itself (Exodus 13:21–22). The cloud did not cover the whole structure, but the “tent of the testimony,” i.e., the enclosure which contained the “ark of the testimony” (Exodus 25:16), (Exodus 25:22), and the holy place. The phenomenon is now again described in connection with the journeyings which are to be narrated later in the book.