According to Success Criterion 1.3.1 (Info and Relationships), radio buttons or check boxes that are part of the same group - that is, using the same name attribute - should be using fieldset elements to group the options as a unit. This allows the author to provide additional context to the labels provided, and associating the options as part of a logical grouping.
This example demonstrates correct usage with check boxes.
The 3 January 2012 update to the techniques document clarified the technique, removing the requirement for a fieldset if the individual labels provide sufficient description, even when the name attributes are identical. It was in conflict with the body text of the description, which suggested that the requirement for a fieldset was not required where the fields themselves "clear instructions and distinct selections".
Failure of this technique is therefore being downgraded from an error to a warning - it is still important for authors to check forms to ensure whether or not a fieldset is appropriate. In many cases it will be.
Where this technique is not required, technique H44, "Using label elements to associate text labels with form controls", should be used instead.