How can they omit those informations??
Likely because they follow the USB Specification. 100 mA without negotioation, 500 mA with negotiation on USB 2, 900 mA with negotiation on USB 3.
Beyond that and for higher voltages than 5 Volts, you need USB-PD. If USB-PD is not specified, don't expect to have it.
These values are the minimum. More is quite likely, especially the 100 mA limit without negotiation is often not followed, but drawing more *may* overload the ports and cause them to shut down. Or, if ASUS cheaped out, gross overloads may even damage the ports, or the controller chip, permanently.