Meaning of $bios_boot and $gptonly when preseeding with Partman

[Tim Herwig <th@sernet.de>]

Hi, I am currently maintaining a PXE server and making it fit for GPT 
and EFI. In the preseed configuration with Partman I noticed the 
following commands, whose meaning I can't find anywhere:
"$gptonly"
"$bios_boot"
They appear in the following context:
             1 1 1 free \
                      $iflabel{ gpt } \
                      $reusemethod{ } \
                      $gptonly{ } \
                      $bios_boot{ } \
                      method{ biosgrub } \
I have already searched the documentation and some source code but have 
not found anything.
Can someone help me with this? Or tell me the right place to look.