If a product is supplied by USB only, what kind of PS class can be considered for that product?
More specifically you asked: What PS class is a USB powered product that has no other I/O connections, has no internal battery nor internal amplifying circuitry? The supplied cable with the product allows connection to any USB A port, however the product maximum power consumption is 700mW. The product is a card reader but could be compared to a USB supplied mouse, from power supply connection perspective.
In response, as noted in sub-clause 6.6 of IEC 62368-1, power delivered to interconnected equipment shall be limited to PS2 or shall comply with Q.1 (LPS). In addition to sub-clause 6.6, IEC TC 108 has issued the first edition of IEC 62368-3, which was published in 2017, and has the title, “Audio/video, information and communication technology equipment - Part 3: Safety aspects for DC power transfer through communication cables and ports.” Its Clause 5 covers power transfer using ES1 and ES2 voltages. USB and PoE are examples of the technologies covered.
However, the requirements in these specific standards do not specifically address your specific challenge since both 62368-1 and 62368-3 consider the system of interconnected equipment – we assume your powered device (PD) is not being considered within a specific system, and you will be asking that it be certified as a stand-alone device (supplied by generic USB Type A).
The concern IEC TC108 has with such situations is (a) USB specifications are beginning to allow for power transfer up to 100 W (the PS2 / PS3 demarcation), and potentially above, and (b) there is USB power sourcing equipment (PSE) on the market that is not designed per the USB.org specification protocol or are counterfeit. Therefore, full confidence that all PSE with USB ports have PS2 outputs is less assured.
IEC TC108 attempted to develop a formal Interpretation for the situation you ask about, but were not able to achieve consensus, in part because the question you raise really is a ‘certification’ issue and not a ‘standards’ issue (as 62368-1 if fully capable of considering the 'system' if the PSE, PD and cabling all are known, and regardless of power level). Therefore, it will be left up to the individual certifier you work with to make a certification-based decision for your situation. To our knowledge, at this time most certifiers currently will continue to assume USB is a PS2 class source for the type of simple scenario you describe, and unless there are special needs of the PD. However, you should check with your certifier, as they may have special certification requirements, such as a special marking or instruction as a certification requirement. For further input from UL on the topic, you are encouraged to contact UL for an in-depth consultation, either via your local account executive or via https://62368-ul-solutions.com/contact-ul.html.