By organizing your property panel into items and allowing them to get groups, you can visually segregate your property panel:

 FontProperties: {
    type: "items",
    label: 'Font Properties',
    grouped: true,
    items: {
      group1: {
         type:'items',
         items: {
            fontSizeSlider: { 
               type: "number", 
               component: "slider", 
               label: "Font Size (px)", 
               ref: "FontSizeSliderValue", 
               min: 10, 
               max: 20, 
               step: 1, 
               defaultValue: 15,
               change: function(prop){     
                  prop.fontSize = prop.FontSizeSliderValue
               } 
            },
            fontSize: {
              type: "integer",
              expression: "optional",
              ref: "fontSize",
              defaultValue: 15
           }
         }
     },
     group2: {
         type:'items',
         items: {
           fontFamily: {
              label: "Font Family",
              type: "string",
              expression: "optional",
              ref: "fontfamily"
           }
         }
     }
 }
}

These will generate the following grouped view in the property panel: