Adam Clarke on Numbers 12:7

Moses - is faithful - נאמן neeman, a prefect or superintendent. So Samuel is termed, 1Sam 2:35; 1Sam 3:20; David is so called, 1Sam 18:27, Neeman, and son-in-law of the king. Job 12:20, speaks of the Neemanim as a name of dignity. It seems also to have been a title of respect given to ambassadors, Pro 13:17; Pro 25:13. Calmet well observes that the word fidelity is often used for an employ, office, or dignity, and refers to 1Chr 9:22, 1Chr 9:26, 1Chr 9:31; 2Chr 31:12, 2Chr 31:15; 2Chr 34:12, etc. Moses was a faithful, well-tried servant in the house of God, and therefore he uses him as a familiar, and puts confidence in him.