none: New schema can be any valid schema

forward: New schema changes are compatible with earlier versions. i.e. if a producer is publishing a newer version of a subject then old consumers should not break because of new changes.

backward: Old schema is still supported i.e. if a consumer is moved to a newer version of schema but the producer is still on the older version then the consumer should be able to support the older version record.

full: New schema is both forward and backward compatible.