Here’s a good example of why you might use the User Options, and how it lets you set Arnold parameters that aren’t exposed by MtoA in Maya.
If you set the verbosity level to Warnings + Info, you’ll get some detailed image statistics in the Arnold log:
But what if you’ve got thousands of image files (eg udims)? You might not want all that in your log. There used to be a check box in the Arnold Render Settings that allowed you to turn off the detailed image file statistics, but we removed that a few years ago.
But the Arnold options.texture_per_file_stats parameter is still there, so you can use the User Options field to set that parameter to false. Just enter the parameter name “texture_per_file_stats” and the parameter value, like this: