Same as SHIPXYx.DAT. All ships are stored. Host uses this file instead of fetching the relevant data from the SHIP.HST file, hence add-ons must update both files. In particular, Host does not update it before doing Ion Storm effects, therefore it uses ship masses from the previous turn.
PHost does not read this file and only writes it for compatibility.
500 (resp. 999) ship records: +0 WORD X position +2 WORD Y position +4 WORD Owner +6 WORD Mass in kt