Why do PoE ports require markings of the electrical output ratings?
More specifically you asked: Re: 62368 Annex DVA (Annex Q) for PoE: As I know, the diameter of the network cable used by PoE port or WLAN port is the same. Even if different voltage and current of PoE port are identified, the same specification of network cable will be used, because the diameter of the network cable is same. If it is understood in this way, the PoE port without identifying voltage and current can also meet the requirements of Annex Q of 62368, so why does our POE port have to mark voltage and current?
In response, the requirement you reference is a regulatory requirement based on NFPA 70 (NEC), Section 725.121(C).
For constructions that will utilize Class 2 or 3 cabling after installation, thus Article 725, this requirement applies to outputs of power sources providing either Class 2 or 3 power per Section 725.212(A)(3), or, as permitted by Section 725.121 (A)(4), limited power (source) circuits of listed AV/ICT equipment. This requirement does not apply to communication circuits not providing power.
Also, it is important to note, per Section 725.144 and Table 725.144 wire gauge sizes of 22, 23, 24 and 26 AWG are anticipated by Article 725, each of which have different current-carrying capacity and temperature considerations, especially if bundled. Therefore, it is incorrect to state that the diameter of all network conductors will be the same.
Additional information can be found in answer to similar question posted on 62368 UL Toolkit: https://62368-ul-solutions.com/engineers/what-are-the-options-for-marking-limited-power-outputs,-e.g.,-poe-ports,-per-the-nec-section-725.121(c)/
For further assistance 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.