If a new function is added to a device then it will most likely require a PMA. If the device remains largely the same but with a slight change you can proceed with a 510(k). For example, to my understanding if a new hip stent is designed and the only change is surface roughness for the goal of bone adhesion this would not require a PMA. However, if the new stent released a drug to prevent infection this would require a PMA since this is a significant change to the device. The FDA lists the following as some of the possible reasons that a 510(k) would not be acceptable:

• A change in indications for use from prescription use to over the counter use
• Addition of a new patient population
• Changes to the environment of use such as from professional use to home use or hospital use to ambulatory transport
• Changes in frequency or duration of use
• Change to indicate compatibility with a type of device, component, or accessory that was not indicated as compatible with the previously cleared device
• Changes in sterilization, cleaning or disinfection
• Changes in package integrity or shelf-life claims
• Changes in device design
• Changes to employ wireless communication
• Changes in the human factors of the patient or user interface
• A change in material type, formulation or chemical composition
• Changes in the antibody, detection reagents, critical reaction components or conjugates for in vitro diagnostic (IVD) devices

While some of these are specific and set clear boundaries (i.e. employ wireless communication, change in material type, etc), it is intentionally left vague as in the extent each change could be. This gives the FDA more room to accept some devices and deny others based on more specific details rather the overall concept.

If a new function is added to an existing class 2 device, than I believe a PMA would be necessary in order to ensure that: 1) The new functionality does not interfere with any of the existing functions of the device and 2) The new functionality can be proven to be safe to use and implement into the device. 

In regards to your other question, I believe a predicate device is a device that has either existed before the 1976 Medical Device Amendments or been approved by the FDA after the 1976 Amendments which serves a similar or the same purpose as the device that is being proposed to the FDA for approval. For example, if you were to propose a hip implant made of a certain material but different design, I believe you can reference previously approves implants with a similar design or the same material as predicate devices. Let me follow that up with my own question using the same scenario: If the same material was used in an implant for a different part of the body, would it still be able to be used as a predicate device?

As listed by both answers, the reason why a 510k would be used is that the main function of the medical device and all its secondary functional parts of the item would have to be the same, and very minor modification would be allowed. This I believe is part of the FDA's initiative to expedite innovation in the scientific community. Because of this a lot of companies use this clause to bypass extensive safety regulations and expensive clinical studies that would need to be conducted by the company. Many people would say that the government is stifling innovation by not allowing for more innovative yet may pose the question of safety. Which brings to the question of the need for doing clinical testing for a class 2 device that introduces a new functionality. I believe that it definitely is required because of the ability for this new functionally to affect the other parts of the device and to make sure there is no interaction that may cause adverse reactions to the whole device. For your question of what a predicative device, that is a device that was previously cleared by the FDA which other devices can use a 510k filing to show that the new technology is very similar to the cleared one. 

Thank you @wonbum-sohn for creating this post and for everyone who has responded. As mentioned in other threads, I am working on a point of care device for early cancer screening. Although the targeted biomarker of interest for the device is similar to current liquid biopsy technologies such as CellSearch, it takes a completely different approach and is intended a clinical test to substantiate indication for further testing and not as a tool for diagnosing patients with specific cancers. CellSearch was filed as a Class 2 medical device before being recalled [1]. I assume it was filed as a Class 2 medical device because the only human interface portion of the device is collecting a peripheral blood sample similar to a glucometer or HIV test, however, a glucometer is considered a Class 2 device, hence requiring a 510(k), while an HIV test is a Class 3 device, which requires Premarket Approval (PMA). I know the FDA has been discussing the idea of potentially reclassifying HIV tests into Class 2 [2], but one of the other major reasons for contention when determining the class of a medical device on Request for Designation (RFD), that has not been mentioned by the other responses, includes the intended use and weighing safety/health risks especially as a diagnostic tool that can influence ones future plan of care. I would appreciate any input on what the appropriate regulatory pathway would be for the technology I am working on.

[1] https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfres/res.cfm?id=163784  

[2] http://hivtestingconference.org/wp-content/uploads/2019/04/Primer-5_Lathrop-FDA-Update.pdf

To clarify from my original post, I would think as per the conditions listed by the FDA for what devices qualify as a Class 2 device and what prohibits devices from filing a 510(k) as per the post from @anthonynjit, that I would need to apply for PMA but CellSearch is a relatively new technology that was able to file as a Class 2 device and I would imagine there are major implications from a false positive test, so I do not understand how this device was not considered a Class 3 device while HIV tests currently are.