"Hot Swap" is where you remove an old device and then plug in a new device to replace the old one. You have thus "swapped" devices. Now in addition to being able to detect that a new device has been plugged in, the removal of the old device needs to be detected too.
External devices that connect to the serial port via a cable (such as external modems) can also be called Plug-and-Play. Since only the serial port itself needs bus-resources (an IRQ and I/O address) there are no bus-resources to allocate to such plug-in devices. In this case, PnP is used only to identify the modem (read it's model code number). This could be important if the modem is a software modem (linmodem) and requires a special driver. There is a special PnP specification for such external serial devices (something connected to the serial port).
Linux doesn't support this yet ?? For a hardware modem, the ordinary serial driver will work OK so there's little need for using the special serial PnP to find a driver. You still need to tell the communications program what port (such as /dev/ttyS1) the modem is on. With PnP you wouldn't need to even do this. With the advent of software modems that have Linux drivers (linmodems), it would be nice to have the appropriate driver install itself automatically via PnP.