Game elements in possession of a Robot are an extension of that Robot. Therefore, Entanglement (e.g., grasping, hooking, attaching) with Cubes that are in the possession of an opposing Robot is a violation of this rule.
From the context of subrule d (talking about grabbing onto cubes that another robot are possessing), as well as the general nature of G12 (limiting defense and destructive behavior), it would make sense that this concept was meant to apply to such situations only.
However, in a recent Q&A answer, the scope of this rule was demonstrated to be significantly larger:
Their robot was in possession (not just touching) of a cube and that cube was touching the base cube of that stack. He felt that the way the rule is written, the stack should not be considered scored since the robot (through that cube in its possession) was touching the base of the stack. If the possession of a cube is an extension of the robot, referee felt the robot was touching the base cube of the stack since it possessed the cube that was touching it.
This is a correct interpretation (assuming that the Robot possessing the Cube was a Red robot). The Cube being contacted would not be considered a Base Cube, and the Scored status of all other Cubes in the Goal Zone would be affected accordingly.
...[these rules] are intended to be applications of an overall principle that Robots and any Cubes they are possessing should be considered as one entity.
With this expansion of the rule, it seems that the cube would be considered a part of the robot in many more situations than appears at first glance. This brings up several questions:
For clarity, a cube which is possessed by a robot will be referred to as a possessed cube
SG3: Would it be illegal for a possessed cube to perform any of the actions under SG3?
a. In particular, if a possessed cube contacts an opponent's Inner Protected Zone, would the offending robot be disqualified?
G4: Is it illegal to have a preload which counts as a possessed cube extend outside of the starting volume?
G5: Is it illegal to intentionally detatch a possessed cube from a robot?
SG3a: Is a possessed cube considered part of the robot for the purpose of determining whether a Robot is considered to be "fully contained" within its Protected Zone?