As Richard indicates so well, Voluntary describes an action.
"Volunteer" exists as both Noun and Verb.
If I am a "Volunteer" the word refers to a person.
If "Volunteer" is what I do, the word describes an action I perform.
For example, as a person who practices English with foreign students without charging them money for the service, my actions are "voluntary".
I am a "volunteer" as a language practice partner.
I "volunteer" my services.
So "volunteer" can refer to (A) What I am, and (B) What I do.
"voluntary" refers to the actions that I do.
We can consider another related form, which is "involuntary".
I can perform "involuntary" actions. For example, many students take exams as an involuntary action. They do not really want to take the exams. They are required to take exams. Their actions, in taking exams, are "involuntary".
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