Hint: Settings for MusicBrowser also determine if pick list can be used

In the settings you can configure which element hierarchy you want to browse in SmartPlaylist's  Music Browser (Menu > Settings, then scroll to the bottom to "MusicBrowser: Show / hide:").

At the moment these settings are also applied in the rule details for Composer, Grouping, Artist, Album and Title. So, if you have deactivated Composer and Grouping as in the example above, you will not be able to pick a composer or a grouping in the details for the Composer rule or Grouping rule, respectively, as the list will be empty.

So, at the moment (SmartPlaylist v7.3.1) you have to activate all levels in the Music Browser in order to use all the pick lists for Composer, Grouping, Artist, Album and Title.
(This was done to minimise waiting time to build the pick lists. But the new iPhones / iPads are now so fast that this is not required anymore.)

In the next version of SmartPlaylist this will no longer be necessary and you can configure the Music Browser as you want and still be able to pick Composer, Grouping, Artist, Album and Title.