Albert Barnes Commentary Acts 19:39

"But if ye seek anything about other matters, it shall be settled in the regular assembly." — Acts 19:39 (ASV)

But if you inquire. If you seek to determine any other matters than that pertaining to the alleged wrong which Demetrius has suffered in his business.

Other matters. Anything concerning public affairs; anything pertaining to the government and the worship of Diana.

In a lawful assembly. This refers to an assembly convened not by tumult and riot, but in conformity with the law. The current gathering was a tumultuous assemblage, and it was proper for the public officer to demand that they disperse. If there were any public grievances to be remedied, they should be addressed in an assembly properly convened.

It should be noted here that the original word rendered assembly is the word usually rendered church in the New Testament. The Greek word is ekklēsia. In this context, it is properly rendered by the word assembly—not denoting a mixed or tumultuous assemblage, but one convened in the legal manner. The proper meaning of the word is that which is called out. The church, the Christian assembly of the faithful, is made up of those who are called out from the world.