The "importance" field in a work request is an indication of importance of the work. It's currently the number after the decimal point in the priority field, expressed in the range [0, 10). E.g. the "importance" in a priority of "4.82" is "8.2".
The smaller the number, the higher the importance. The default is 5.
When specifying this field in a customized sort or display, any of the names "importance" or "Importance" can be used.