SIGMA DP1 - Manual (Page 87)

86
TO MARK A SINGLE IMAGE
1
Use the  button to select the image to be marked in Contact Sheet or
Single-Image View.
2
Use the  button to open [ Playback Menu ].  
3
Use the  button to select [Mark] and press the  button. 
4
Use the button to select [Mark].
5
Press the  button or the  button to apply the settings or the 
button to close the sub-menu without making any changes.
A icon will appear in the upper-right corner of the image indicating that
the image is marked.
UNMARKING A SINGLE FILE
To unmark a marked image, select the marked image then select [Unmark]
in step 4 and press the
button.