“Set” refers to mindset, which is reflected in one's attitude, thoughts, beliefs, expectations, and emotions.
Optimize the client's "set" with good preparation, education, intention-setting, and mental priming.
"Setting" refers to the environment the client is in.
Optimize the client's "setting" by providing a supportive, facilitative, and non-distracting environment.
Remember that YOU are part of the client's setting.