From first hand experience, see signature, it is the exhaust brake. You turn the switch on and when you take your foot off of the accelerator the exhaust brake comes on. Even if you have a manual you might not learn what some switches do. I found my manual to be generic with some things missing. Do you have air bags or springs? If the latter you have a Freightliner XCS chassis like mine. I have had mine for 11 years since new and it is a great chassis. Let me know if you have other questions.
2014 Palazzo 33.2
Cummins 6.7 ISB, Allison 2100 6 Speed