Hello Bentley,
This idea is either for an additional fixed-width column when selecting Elements by-Level, or the existing [is used] column to display whether a level is used or not.
A fixed width column to sort by used/not used.
Click on the column to sort the list up/down.
makes it easier to find populated Levels for Selection Sets.
There is already an existing second passive column that only displays whether a Level is populated or not, but this only updates after selecting Elements.
To be able to sort Levels, first by population, would be helpful.
In this mock up, the Select By palette has a dot that appears to show which attribute/property is currently in use.
In its current form, the dot is populated after selecting Elements.
With this feature in mind, I can sort the list based on populated items instead of guessing.
Would save a few clicks and some guesswork.