Albert Barnes Commentary 1 Chronicles 6:54

"Now these are their dwelling-places according to their encampments in their borders: to the sons of Aaron, of the families of the Kohathites (for theirs was the [first] lot)," — 1 Chronicles 6:54 (ASV)

Theirs was the lot - that is, “the first lot.” The Kohathites had the first lot among the Levitical families, as they were the family to which the high priesthood was attached .