As far as I have tried you cannot access the fields separately.
All the data is kept in category>spider>info
and cover is kept in category>spider>cover
There are two commands
spider save which saves only the coverart
and spider save info which saves all the other data but coverart.
This is done so that you can get movie information from one source (imdb) and coverart from other (amazon)
Look at this post
http://www.xlobby.com/forum/viewtopic.php?t=2841