It is clearly explained in the manual that this is because you have royalty reporting switched to on. This is how it gathers the data for the report:
Royalty reporting capability
If enabled, the autoDJ will include a small tracking code at the end of the song titles shown in your listeners' media players. For example, instead of:
Artist Name – Song Title
You might see the following instead:
Artist Name – Song Title [C51B]
This allows detailed reports to be generated (on the Tracks tab of the View Statistics page) containing most of the information needed to make reports to the appropriate entities for royalty determination purposes.
If this option is disabled, the generated reports will be much less detailed and may not contain sufficient information for accurate royalty reporting purposes.