The default  primary sorting in the Spectral Line Picker is by Rest Frequency.

In order to change the primary sorting column, it is necessary to first double-click the desired column. This will cause a triangle to appear in that column's heading and subsequent single clicks will cause the sort order, indicated by the direction of the triangle, to change. 

Note that it is also possible to refine the sort with secondary and tertiary, etc., criteria by single-clicking on additional columns - smaller triangles will appear in the column headings and, as with the primary sort, further single clicks will change the sort order of that column. For example, double-clicking first on Description will produce many examples of the same molecule.

A secondary sort can then be performed on the Rest Frequency and, if there are multiple entries with the same Rest Frequency, a further sort might be possible with the Sij \mu^2 column (line strength multiplied by the square of the dipole moment). The sort can be reset by double-clicking on a column i.e. only this primary sort will now be enabled. By default, the OT performs a primary sort using the Rest Frequency and a secondary sort using the Transition column.

For more information, please see the Spectral Line Picker chapter of the OT User Manual.