Optional (adjective)

Not compulsory; left to personal choice or preference.

Origin:

From option +‎ -al, from latin optio (“choice, option”).

Examples:

1. Attendance at the meeting is optional.
2. The restaurant offers optional extras such as dessert and wine.
3. The course has a few optional modules.
4. The purchase of insurance is optional for this product.
5. Some airlines charge for optional services like checked baggage and seat selection.