'Voluntary' in the Bible

"You won't be allowed to eat your tithe of grain, new wine, oil, the firstborn of your herd and flock, your voluntary offerings that you pledged, your free-will offerings, and the works of your hands in your own cities.

Then you are to celebrate the Festival of Weeks before the Lord your God with the voluntary offering that you will bring, in proportion to how he has blessed you.

You shall be careful to perform that [vow] which passes your lips, just as you have made a voluntary vow to the Lord your God, just as you have promised with your own words (mouth).