Answer

Solo In Place (SIP) is a handy function for system set up. 

SIP mutes all the mixer inputs except the input (or inputs) that is (or are) soloed. The "Solo In Place" part means that the channel is still routed to wherever it was routed before SIP was engaged. It is used as an alternative to manually muting all the channels other than the one the operator is working on. SIP mode may be selected / deselected from the Mixer Setup page.