Once upon a time, life was relatively simple in the x86 world. Most x86 compatible PCs theoretically had one or two UARTs, which were called COM1 and COM2 by MS-DOS and Windows, ttyS0 and ttyS1 by Linux, 'ttyu0' and 'ttyu1' by FreeBSD, and so on, based on standard x86 IO port addresses for them. Servers had a physical serial port on the back and wired the connector to COM1 (some servers might have two connectors). Then life became more complicated when servers implemented BMCs (Baseboard management controllers) and the IPMI specification added Serial over LAN, to let you talk to your server through what the server believed was a serial port but was actually a connection through the BMC, coming over your management network.

Early BMCs could take very brute force approaches to making this work. The circa 2008 era Sunfire X2200s we used in our first ZFS fileservers wired the motherboard serial port to the BMC and connected the BMC to the physical serial port on the back of the server. When you talked to the serial port after the machine powered on, you were actually talking to the BMC; to get to the server serial port, you had to log in to the BMC and do an arcane sequence to 'connect' to the server serial port. The BMC didn't save or buffer up server serial output from before you connected; such output was just lost.

(Given our long standing console server, we had feelings about having to manually do things to get the real server serial console to show up so we could start logging kernel console output.)

Modern servers and their BMCs are quite intertwined, so I suspect that often both server serial ports are basically implemented by the BMC (cf), or at least are wired to it. The BMC passes one serial port through to the physical connector (if your server has one) and handles the other itself to implement Serial over LAN. There are variants on this design possible; for example, we have one set of Supermicro hardware with no external physical serial connector, just one serial header on the motherboard and a BMC Serial over LAN port. To be unhelpful, the motherboard serial header is ttyS0 and the BMC SOL port is ttyS1.

When the BMC handles both server serial ports and passes one of them through to the physical serial port, it can decide which one to pass through and which one to use as the Serial over LAN port. Being able to change this in the BMC is convenient if you want to have a common server operating system configuration but use a physical serial port on some machines and use Serial over LAN on others. With the BMC switching which server serial port comes out on the external serial connector, you can tell all of the server OS installs to use 'ttyS0' as their serial console, then connect ttyS0 to either Serial over LAN or the physical serial port as you need.

Some BMCs (I'm looking at you, Dell) go to an extra level of indirection. In these, the BMC has an idea of 'serial device 1' and 'serial device 2', with you controlling which of the server's ttyS0 and ttyS1 maps to which 'serial device', and then it has a separate setting for which 'serial device' is mapped to the physical serial connector on the back. This helpfully requires you to look at two separate settings to know if your ttyS0 will be appearing on the physical connector or as a Serial over LAN console (and gives you two settings that can be wrong).

In theory a BMC could share a single server serial port between the physical serial connector and an IPMI Serial over LAN connection, sending output to both and accepting input from each. In practice I don't think most BMCs do this and there are obvious issues of two people interfering with each other that BMCs may not want to get involved in.

PS: I expect more and more servers to drop external serial ports over time, retaining at most an internal serial header on the motherboard. That might simplify BMC and BIOS settings.