SHELL in Mail Merge code

[Alex McMurchy]

This Mail Merge code

>     const bool bMT_EMAIL   = rMergeDescriptor.nMergeType == 
> DBMGR_MERGE_EMAIL;
>     const bool bMT_SHELL   = rMergeDescriptor.nMergeType == 
> DBMGR_MERGE_SHELL;
>     const bool bMT_PRINTER = rMergeDescriptor.nMergeType == 
> DBMGR_MERGE_PRINTER;
>     const bool bMT_FILE    = rMergeDescriptor.nMergeType == 
> DBMGR_MERGE_FILE;

appears in sw/source/uibase/dbui/dbmgr.cxx

I'm mostly familiar with the EMAIL, PRINTER and FILE and when and how to 
test/use them.

What I'm not familiar is SHELL. What is it in respect of Mail Merge and 
under what circumstances would I use if and how?

Regards

Alex