What do you mean “What do you mean?” Are you asking me what I mean or is your question “What is meant by …..?”
When a client wishes to change auditors, the proposed replacing auditors should write to the current auditors to see if there is any reason why the proposed new ones should think twice before accepting the invitation to be appointed
The letter that they new ones write to the existing ones is called by me “a professional reasons letter” but is called by Kaplan (apparently, according to your post) a “professional clearance and etiquette letter”