Charles Ellicott Commentary 1 Chronicles 12:31

"And of the half-tribe of Manasseh eighteen thousand, who were mentioned by name, to come and make David king." — 1 Chronicles 12:31 (ASV)

Which were expressed by name. —See the same phrase, 1 Chronicles 16:41; Numbers 1:17. Literally, it is marked down, or entered in a list, by names. The men had been levied by the tribal chiefs, and enrolled in lists for this particular service.