11 U.S. Code § 1122. Classification of claims or interests
This section codifies current case law surrounding the classification of claims and equity securities. It requires classification based on the nature of the claims or interests classified, and permits inclusion of claims or interests in a particular class only if the claim or interest being included is substantially similar to the other claims or interests of the class.
Subsection (b), also a codification of existing practice, contains an exception. The plan may designate a separate class of claims consisting only of every unsecured claim that is less than or reduced to an amount that the court approves as reasonable and necessary for administrative convenience.