Unknown bytes. When parsing the ACT file 16 bytes will be read from the buffer directly into a class (not struct) that holds the three members x, y and attr. Funny that this actually works.
Offset x coordinate of this attachment.
Offset y coordinate of this attachment.
Apparently this value is used to compare against another attachment point when stitching together the separate sprites (e.g. body and head). However, only the first attachment point of each sprite is used for the comparison. For that to work it implies that this value is always the same (in this case 0).
From my research years back most of the info comes from "deufeufeu" around 2003. Who shared info about the SPR and ACT file format. His work has been archived by Gratia Huang on his website: http://mist.in/gratia/ro/spr/deufeufeu_files/ (Archive). Later on I've been fortunate to have been gotten a hold of the source code of a chinese game called DHXJ (东方仙境). Special thanks to the ASB community for that. This game is based on the pre-renewal RO client but without knowing what kind of modifications have been made the info it provides should be looked at critically.