The class has three sub-types and they are distinguished by what prompted the offering rather than by anything visible in the rite. The thanksgiving offering answers a deliverance that has already happened. The vow offering discharges a promise made in advance, typically under duress. The freewill offering answers nothing at all: it is brought because the offerer chose to bring it, and the tradition treats that as a category in its own right rather than as an unmotivated version of the others.
The thanksgiving offering alone comes with bread, four kinds of it, unleavened cakes mixed with oil, unleavened wafers spread with oil, cakes of soaked flour, and, uniquely on this occasion, leavened loaves, which are not burned on the altar but given with the animal. One of each kind goes to the priest who dashes the blood. That the one class of offering associated with public acknowledgement of rescue is also the one that comes with leavened bread and a same-day deadline says something about the kind of occasion it was: a household with a great deal of meat and bread that had to be eaten before nightfall, and therefore a great many guests.
The freewill offering carries one relaxation the others do not. An animal with a limb too long or too short may be accepted for it, though not for a vow, which is the only place the system bends its requirement of an unblemished victim. Prophetic and narrative texts mention this class more than any other, because it is the one that ordinary people brought and ate, and the one that made a pilgrimage festival a festival rather than a duty.