The smb.conf man page describes the %D variable as "Name of the domain or
workgroup of the current user." When used in smb.conf, though, it seems to be
returning the NetBIOS name of the server. For instance, in a file share on a
server with a NetBIOS name of NESSUS on the RINGWORLD workgroup:

preexec = touch /tmp/%D

After accessing the share from a remote system on the RINGWORLD workgroup (using
smbclient or smbmount), a file called /tmp/NESSUS appears, whereas based on the
smb.conf man page, I'd expect a file called /tmp/RINGWORLD (or perhaps
/tmp/ringworld) to be present.