This subject is optional [《米》 elective].／You have the choice [option] of taking this subject or not.
This money is at your disposal [discretion].／You can use this money as you please [like].
You are free to give any interpretation to it you like.
a voluntary donation
I was requested to make a voluntary appearance at the police station.
The police searched the house of the witness with his consent.
a private organization neither controlled nor protected by law
I (must) ask you to come with me to the police station voluntarily [of your own accord].