I have no idea what they are referring to, but I can tell you that sometimes hammer forged barrels will have 'chatter marks' in the lands and/or grooves which are the result of how the hammers and mandrel are set up/oriented relative to one another.
Check the papers I linked to here:
https://www.24hourcampfire.com/ubbthread..._button_vs_good_hammer_forge#Post6325697 
