This may entirely rely on the industry that the class II device is within and may vary on how well the industry is able to prove efficacy and safety of their device. Class II devices have a large subsection that defines which category it stands; since the categories are so broad, devices like powered wheel chairs almost definitely does not a clinical trial but devices like a knee prosthetic may. This all depends on asking the question on the efficacy of the device. If there is a new material or multi-materials that haven't been verified previously in a clinical/animal trial or in a biocompatibility test, then the regulatory body may not accept the submission as a pre-market notification and one would need to submit human trial data to ensure safety. Sometimes, there are other factors such as the purpose of the device that has a low tolerance towards errors that would cause one to worry about human life. Again, this all boils down to the ability of the device to provide safe treatment to a patient and how of a risk to the patient's safety during use it is. If one were to question if this would be high risk compared to the products currently on the market, then a respondent from the regulatory body would most likely require a clinical trial. 

Like you said, the characteristics and requirements for Class I and Class III medical devices are quite clear-cut. Class II devices are a little more complicated as these devices are often times a combination of the two classes, leaning slightly more toward Class I or Class III depending on the amount of risk that the device poses to the user. Navigating this middle ground requires more judgement and critical analysis from the FDA. In general, Class II devices are an improvement of an already existing device rather than a new invention that can potentially fill a gap in the medical device market. This is the main distinction between a Class II and Class III device. However, to make things more complicated, both Class II and Class III devices can undergo clinical trials. Class II devices require clinical trials when failure of the device can result in severe injury to the user, whereas Class III devices almost always require clinical trials. All in all, the easiest way to define a Class II device is any medical device that poses a moderate amount of risk and that is subject to general and special controls. Some examples: orthopedic implants, absorbable sutures, MRI scanners.

Medical devices are labeled based on there efficacy and safety of the product being developed. Clinical trails being conducted on medical devices is determined based on the possible threats that these classes could pose on humans, or the subjects. These trials are different between classes as well based on their design and execution. If the Class 11 device presents a possible risk, then a clinical trial will need to be conducted.

The classes are ranked according to their risk, or their safety and efficacy as mentioned above.  Class 2 medical devices are those that have moderate to high risk to patients. The only thing I really found on Class 2 medical devices is that some of them are required to go to trial by the US. It depends on how high risk they are.  The higher the risk of the device, the more it needs to be studied in clinical trials. 

Most medical devices are viewed as Class II devices. Class II devices are those 43% of clinical gadgets that have a moderate risk which general controls are not adequate to guarantee wellbeing and viability to the patient. The FDA has distinguished product code-explicit guidance records and Recognized Consensus Standards as a feature of their special controls. Instances of Class II devices incorporate OTC blood glucose checking systems, scalp cooling systems, controlled wheelchairs, and some pregnancy test. Gadgets in this class may apply for a 510(k) exemption and be added to the list of PMA exempt devices. This move is planned to assist and simplify the way toward getting gadgets that don't represent a critical danger on the market sooner. I'm certain that the process of class II gadgets fairly correlates to class I as far as going from pre-IDE to IDE then PMA.