
optional
Available as a free choice; not strictly required or prescribed.
Ein Dessert ist im Menüpreis enthalten, die Käseplatte ist optional.
A dessert is included in the menu price; the cheese platter is optional.
Loading your saved state.

Available as a free choice; not strictly required or prescribed.
Ein Dessert ist im Menüpreis enthalten, die Käseplatte ist optional.
A dessert is included in the menu price; the cheese platter is optional.
The adjective 'optional' belongs to the word family of 'die Option' (the option). If you have an option, meaning a choice, then that thing is optional.
Die zusätzliche Versicherung für den Mietwagen ist optional.
The additional insurance for the rental car is optional.
'Optional' can stand as an adjective before a noun ('ein optionaler Kurs' — an optional course) or as an adverb referring to the whole sentence. As an adverb, it often stands at the beginning of the sentence or next to the verb.
Die Teilnahme an dem Workshop ist optional.
Participation in the workshop is optional.
The word is frequently used with offers, services, or parts of a product. Typical combinations are 'optionales Zubehör' (optional accessories), 'optionales Fach' (optional subject), 'optionaler Ausflug' (optional excursion), or 'optionale Ausstattung' (optional equipment).
Das Auto hat viele optionale Extras, die den Preis erhöhen.
The car has many optional extras that increase the price.
voluntary
Freiwillig describes a person's decision to do something without coercion. Optional, by contrast, describes the status of an offer or component: it is possible but not required. For participation, both terms can fit, but they emphasize different aspects.

Die Teilnahme an dem zusätzlichen Kurs ist für die Mitarbeitenden optional.
Participation in the additional course is optional for the employees.
Die Mitarbeitenden nehmen freiwillig an dem zusätzlichen Kurs teil.
The employees are voluntarily taking part in the additional course.