There is a two-digit code to identify the ROTSE instrument from which a given image originated. For ROTSE-III, these digits will be either ``3a'',``3b'',``3c'', or ``3d'', depending on which of the four ROTSE-III telescopes is appropriate. The choice is specified in the camerad.conf file (Section 3.5). If more ROTSE-III telescopes are implemented, the extension of the sequence is obvious.
The index number simply distinguishes between images taken on the same day, for the same reason, of the same field, and with the same telescope. This number increments with each image taken as part of a given imaging sequence (Section 4.4.1) in response to a given command. For sky patrol images, the number increases with each image taken of a particular field on a particular night. For burst alerts, this is simply a chronological numbering of all images taken in response to the same alert.
The median dark images described in Section 7.2.2 do not have index numbers after the instrument designation, but the individual images that make up a dark run will be ordered in the sequence in which they were taken.