Specifies canonization type.
Canonicalization prepares the email for the signing or
verification algorithm. It does not change the transmitted data in any way.
There are two types of canonization. A ‘simple’ algorithm that tolerates almost no modification and a ‘nofws’ algorithm that tolerates common modifications as white-space replacement and header line re-wrapping.
For more information see draft-delany-domainkeys-base-01 (Mark Delany), part 3.4.