Hi Tw,
XACCHD is the structure (working area) in the interface in the userexit. It is defined as ACCHD (you can see the fields in tcode SE11) as XACCIT is related with ACCIT (see it with SE11 again). It is a structure with the values during the interface to accounting from billing, and with some enhancement we have the option to change some values according requirements don't fit with customizing options, for instance, values for PRCTR, values for SGTXT, BKTXT and so on.
Regards
Eduardo