abstract val participants: List<AbstractParty>
A participant is any party that should be notified when the state is created or consumed.
The list of participants is required for certain types of transactions. For example, when changing the notary for this state, every participant has to be involved and approve the transaction so that they receive the updated state, and don't end up in a situation where they can no longer use a state they possess, since someone consumed that state during the notary change process.
The participants list should normally be derived from the contents of the state.