That filed "SL Updates" refers to the Progressive Rendering.
This has been designed mainly for animations.
This field allows you to set the SL steps at which you want yo get results of your scene.
I.e: Setting your SL Updates to 3, 7 and 12, makes Maxwell to render your whole animation to SL=3, and get that "noisy" version pretty fast in your output folder.
You can start the postproduction of your sequence with that rough version. Meanwhile, Maxwell continuos working, resuming your sequence from SL3 to SL 7, and provides you refresh of your renders while you are postproducing them.
Finally, it resumes the whole sequence from SL7 to SL12 (in our example) to provide you with the final quality, but allowing you keep working on the following processes.
This Progressive Rendering workflow represents a huge saving of time, as the render process can happen overlapped during your postproduction time.
There is a more detailed description of this feature here:
http://support.nextlimit.com/display/ma ... g+Workflow
Let me know if this answers your question.
Dario Lanza