It seems the decision is ultimately in the hands of the Authority Having Jurisdiction (AHJ), as long as "equivalent objectives can be achieved" using the device.

National Electrical Code 2008

ARTICLE 100 Definitions

Approved. Acceptable to the authority having jurisdiction.

Authority Having Jurisdiction (AHJ). An organization, office, or individual responsible for enforcing the requirements of a code or standard, or for approving equipment, materials, an installation, or a procedure.

ARTICLE 110 Requirements for Electrical Installations

110.2 Approval. The conductors and equipment required or permitted by this Code shall be acceptable only if approved.

ARTICLE 90 Introduction

90.4 Enforcement. This Code is intended to be suitable for mandatory application by governmental bodies that exercise legal jurisdiction over electrical installations, including signaling and communications systems, and for use by insurance inspectors. The authority having jurisdiction for enforcement of the Code has the responsibility for making interpretations of the rules, for deciding on the approval of equipment and materials, and for granting the special permission contemplated in a number of the rules.

By special permission, the authority having jurisdiction may waive specific requirements in this Code or permit alternative methods where it is assured that equivalent objectives can be achieved by establishing and maintaining effective safety.

This Code may require new products, constructions, or materials that may not yet be available at the time the Code is adopted. In such event, the authority having jurisdiction may permit the use of the products, constructions, or materials that comply with the most recent previous edition of this Code adopted by the jurisdiction.

WARNING

While these devices may or may not be approved by the AHJ, they are not the best solution available. They do not prevent the user from overloading the generator, since the load selection is made by the user dynamically during operation. The better, safer solution, is to install manual or automatic transfer switches. This allows you to pre-select the loads making it a more user friendly solution.

Part of the confusion is 'compression' is sometimes applied to fitting types that are not truly compression type. The term should only be applied to fittings where the compressive force is is applied by screw threads via part of the fitting itself as opposed to compressive force being applied by external tools.

In this compression fitting, via the threads seen on the right of the left hand piece and inside the right hand piece, the tapered ring on the copper tube is compressed, squeezing the tube tightly such that leakage is not possible. Plastic tube variants have an additional insert piece that fits in the end of the tube so it does not collapse under the compressive force.

This is an example of a crimp fitting. The ring is slipped loosely over the tube before pushing the tube onto the barbed fitting.

An external tool is used to apply the compressive force to the crimp ring. The material properties of the crimp ring are plastic, so that when the tool is removed, the ring continues to apply significant compressive force on the tube. Note that this tool is carried in the field to facilitate on-site assembly of piping systems.

A swage fitting is yet another type. It is similar to a crimp fitting in that an external tool is used to apply pressure and a sleeve maintains the pressure after the tools is removed.

The major difference is the compressive tool applies much much more force to the sleeve than a crimp tool. A swage tool is typically not very portable and not used in the field. Rather, pipes are pre-fabricated in a shop with a heavy duty hydraulic machine. The types of fittings swaged to pipes typically allow field assembly using common mechanic's tools.