They're reference points so you can see the position of your accessory in relation to the avatars skeleton during construction.
Basically when the tail was built, the only points (avatar bones) needed were the two you mention; 1) "Female03MasterRoot", so the 'root' of the avatar is visible at all times - knowing this helps orientate objects when being made; and 2) "Pelvis", the avatar bone to which the tail is attached.
For example, if the accessory were a bangle or bracelet, the 'Pelvis' reference would be positioned and orientated to match the avatars 'wrist' (or rather, the bone corresponding to the 'wrist') and renamed appropriate (for clarity) before then using the two 'attachmentNode' bones in a similar way to making the tail - they are positioned at the appropriate bone and any accessory bones are then attached to them.
It should be noted the object used for the two bones are just simple mesh cubes primitives rather than being actual usable 'bones', their purpose is for 'reference' only so there is no need for them to be actual Armature Bones.
