Option flags for custom properties (bpy.props) I just read about properties in the documentation. Every type of property can be created with some options. Most of options are self explanatory, but I really don't know how to explain the option called 'PROPORTIONAL'. Example:

bpy.types.Object.example = bpy.props.IntProperty(name='Example', \
    default=-1, options={'PROPORTIONAL'})


I was able to find the following doc string:


  Adjust values proportionally to each other


but it does not make much more sense. Properties can't be edited using proportional editing, right?

Is there any documentation for all of these option flags?
HIDDEN, SKIP_SAVE, ANIMATABLE, LIBRARY_EDITABLE, PROPORTIONAL?

I wasn't able to find any.
The PROPORTIONAL flag is not related to Blender's Proportional Editing feature, but Multi-Number Button Editing. It's broken in 2.72 for custom properties however. 

There doesn't seem to be any documentation in the API docs about this particular option beside what you already found. The corresponding is_... properties seem to be the only documentation.


HIDDEN: True when the property is hidden.See also Hide custom properties in the UI panel with python
SKIP_SAVE: True when the property is not saved in presets. See also this comment
ANIMATABLE: Property is animatable through RNA (should actually be true even if not specified)
LIBRARY_EDITABLE: Property is editable from linked instances (changes not saved)
ENUM_FLAG: Property is an EnumProperty and multiple entries can be selected (ShiftLMB)