Definition of 'base symbol'

[MARIA GALEA]

Hi everyone,
Has anyone come across the definition of a base symbol - in the literature
or anywhere else? How would you define the base symbol of ISWA 2010? To
define it I'm explaining the whole system of rotations and orientations
for each given base symbol- but still how do you explain the base symbol -
does it represent NO rotation and orientation? (because actually it is
marked for rotation and orientation).
Complex question maybe..sorry- would appreciate any feedback.
thanks
Maria