The light cross (X) in the example you are showing is indicating an "intersction" snap but away from the element. It is like an extension from the physical geometry or a face being "tracked".
The blue dashed line is the logical extents of the elemetn selected. There is a bounding box placed around elements. The bounding box is shown dashed. In most cases the bounding box does not have much effect on behaviors, but sometimes the bounding box will affect something. For example the bounding box of an element controls how an attached end leader extends from an object.
Jeff Hanson Sr. Subject Matter Expert Autodesk, Revit User Experience