For counterfactual nonrealism, it’s simply the uncertainty an agent has about their own action, while believing themselves to control their action.
For policy-dependent source code, the “different possibilities” correspond to different source code. An agent with fixed source code can only take one possible action (from a logically omniscent perspective), but the counterfactuals change the agent’s source code, getting around this constraint.
For counterfactual nonrealism, it’s simply the uncertainty an agent has about their own action, while believing themselves to control their action.
For policy-dependent source code, the “different possibilities” correspond to different source code. An agent with fixed source code can only take one possible action (from a logically omniscent perspective), but the counterfactuals change the agent’s source code, getting around this constraint.