The Details window lets you precisely control the modeling parameters of Vintage Clav and also provides global parameters that affect the overall instrument. See these sections for details:
Vintage Clav Excite and Click parameters
Vintage Clav String parameters
Vintage Clav Pitch parameters
Vintage Clav Misc parameters
Download the guides:
MainStage User Guide: PDF
MainStage Instruments: PDF
MainStage Effects: PDF