This seems to imply that you can choose between an xres, yres of 72 DPI or 36 DPI.
Image span seems to be related to importing images from other programs.
Others Tab- used to lock down and link image dimensions, so they stay constant from project to project, and can't be changed, I'm assuming...though they don't seem to be workinghttp://wiki.synfig.org/wiki/Focus_Point
- related to paste canvas (encapsulated) layers
From the above wiki
"It has a Vector value. It defines which point in the encapsulated layers will stay still as the Paste Canvas layer's Zoom Parameter changes."
Oh well, I see no need to change any of these settings...