What's the difference between Lao Zhang and Master Zhang?

The difference between Lao Zhang and Master Zhang is the distance. Lao Zhang approaches you on behalf of the other party, and both parties can talk at will. It is easier for you to address each other like this. Master Zhang said that the other party has something worthy of your respect, whether it is character, technology or ability, or the other party is older than you. In this case, you can call the other person Master Zhang.