When you are generating a form group for use on IMS™, the mfsExtendedAttr build descriptor option indicates a default value for whether the devices support extended attributes and for whether a color default (CD) extended attribute is generated for form fields that are defined with color = default color.
The information in this build descriptor option is included in EGL-generated MFS DFLD statements; but your decision can be overridden by the mfsDevice option (if any) for a specific device.