Using basic capabilities : Overview of using basic capabilities : Symbols and definitions : Handles
  
Handles
When a symbol is selected, a series of small solid squares, called handles, is placed around the symbol’s bounding rectangle.
The point at which a line is attached to another symbol is displayed as a cross handle, rather than a square handle. This visual clue is useful if you are not sure whether a line is attached to its target and source symbols.
See also
Symbols and definitions