Figures 28-31 mentions goals being open for 'De-scoring' , and triballs being 'safe', words which are not otherwise defined in the game manual. However <SG8> only mentions 'breaking the plane' of the goal:
This is correct. Encroaching on the space within an opponent's Goal ("breaking the plane") is the primary physical action that a Head Referee will be watching for to determine whether an SG8 Violation has been committed.
Non-defined mentions of "de-scoring" and Triballs being "safe" are meant to communicate the overarching intent behind SG8. This intent is to provide some level of protection for the space inside of Goals, and any Triballs contained in that space.
It is possible to cause a tribal to no longer be scored <SC3> without the robot breaking the plane of the goal. Is this legal when the opposing robots are not double zoning?
There are no rules explicitly preventing the action of causing a Triball to no longer be Scored in a Goal when not in a Double-Zoning situation.
However, as explained above, this is one of the intents of SG8. Therefore, any actions which directly or indirectly de-score Triballs from a Goal should be expected to see heavy levels of scrutiny from Head Referees. Interacting with an opponent's Goal in any way is an inherently defensive action, so Teams attempting such strategies should not expect any "benefit of the doubt" if they involve judgment calls relating to rules such as <S1>, <S3>, <G3>, <G6>, <G7>, <G15>, <G17>, and/or <SG5>.
