Save Selection As is available once you have defined an area with the Selector tool on the palette. For information on using the palette, refer to "Using the Image Tool Palette". To save a portion of the image:
Open the image.
For information on opening an image, refer to "Opening Image Files and Documents".
Select a portion of the image to be saved.
For information on using the selector tool to select a portion of the image, refer to "Selecting An Area to Be Saved".
Select File -> Save Selection As.
Saving a selection is the same as using Save as. Refer to "Save As" for complete information.