Ok... I understand but how do I know where the tunnel is on the maps where its use is allowed? It does not appear on the map nor do I see anything that directs me to its location; I've spent a lot of time looking for it...
Yes, you will see it on the map, you just might not have realized it. It doesn't have an icon like a vehicle, but when you're playing as NVA, you'll see it on your map as another spawn point. So if you see two spawn points close together at the beginning of a match, you've found it. Keep in mind that some tunnel spawns don't appear in singleplayer, so if you're looking for it there, you might not find it. On the map, it looks like this:
Of the two spawn points close to each other, the left one is the tunnel spawn. The spawn itself looks like this:
I don't play online much, but according to the map files, the tunnel spawn can be found in multiplayer on:
Ho Chi Minh Trail: Near the northern-most flag.
Cambodian Incursion: TWO tunnels spawn, again near the northern-most flag.
Ia Drang: On the NVA artillery hill, the flag closest to the US main base.
Siege of Khe Sanh: On the NVA artillery hill to the left of the US main base.
Landing Zone Albany: Multiple tunnels in multiple locations, BUT they only spawn when the US holds the flag each individual spawn is tied to.
Fall of Lang Vei: Down the hill from the NVA main, right next to where the ZSU spawns.
Operation Hastings: At the NVA main, to the left of where the MiG-17 spawns.
Operation Irving: Like Cambodian Incursion, two tunnels spawn. One is near the village flag, and the other is in the temple ruins.
If you want to play around with it on singleplayer, the only maps you'll find it on are Landing Zone Albany (but again, the spawns are tied to certain flags and will only spawn when the US holds those flags, so you won't see them at the start of the match) and Operation Hastings. The spawn on Hastings has a 2-second spawn delay at the beginning of the match, so the spawn icon won't show up immediately. It's also in a very slightly different place than it is on multiplayer, but still to the left of the MiG-17. It's actually a bit easier to see than in multiplayer if you don't spawn right on it.