It says set to -1 to not use region tags and other thing
Viewing post in [CGMZ] Footsteps for RPG Maker MZ comments
I guess maybe it is a bit confusing, what it means is that you set it to -1 if you don't want it to check for the region or terrain tag. For example if you want your footstep to occur on region 1 and not look at the terrain tag, you would enter in 1 for the region id and -1 for the terrain tag. Or if you want it to occur on terrain tag 0 and not look at the region you would set terrain tag to 0 and region id to -1.