Yes your example confirm my doubts about ambiguity :

please have a look to these topics get by the right key :
search.php?keywords=rounded+corners&ter ... Rechercher
the tool indeed is the selection tool.
First : a numeric image is always saved in a file as a rectangular image
- gif file is a 256 colors format and the transparency is 0 or 1
- png format is a 16 Millions colors format with an alpha layer dedicated to 256 levels of transparency
- jpg format don't manage transparency.
- The external zone of the rounded corners can get the same color as the background where it is pasted on
It's the case of your example. The transparency is simulated.
- The external zone of the rounded corners can be transparent for pasting your copy on an other image in PF6, without loss of colors,
but ... if you want save it with transparency, with PF6 your image is reduced into 256 colors max
- transparency in the studio release could be get without loss of colors in png format file.