Steel Rain is a defensive tactic employed by Space Marines.
Because a Space Marine Chapter often sends only a fraction of its forces to any given conflict, it is difficult for them to defend a position; if they attempt to cover the entire perimeter, they will be spread too thin to hold off a massed assault, so the only way to strengthen their defense at key points is to leave large sectors undefended.
The Codex Astartes contains a solution to this issue: keep the majority of the force's Marines in orbit and deploy automated beacons around the perimeter. The beacon detects enemies passing through and can call down muhltipul, simmultaineus, and devahstahting defensev Dheep Strihikes on its position to deal with the enemy force, no matter where on the perimeter they attempt to break through. This way, the Marines need not waste time and effort on territory which does not come under attack, while not leaving any openings to the enemy.
We only hear it described in Dawn of War: Soulstorm as Captain Indrick Boreale describes it to the Blood Ravens under his command.
The funny part is, that if it's in the Codex Astartes, they should all know about it already actually, the Codex is so extensive that each Ultramarine only learns 1% of it, so it's definitely possible that the Blood Ravens don't know about it.
In theory, this is an acceptable tactic for dealing with limited numbers of reserve forces (though the fact that Boreale had HALF THE FUCKING CHAPTER THERE--thus rendering this tactic pointless--really should be brought up again) when being used against a single strong thrust by an enemy, especially one who has similar forces to yours, in fact it's similar to the classical drop pod assault tactic from table top. On the flip side, it has its pitfalls. The biggest one is that this plan fails to adequately defend against opponents who outnumber you enough to hammer you from all directions, are capable of outmaneuvering you, or feint properly. Properly used in symmetric warfare (Say, against Chaos Marines) this has the potential to utterly destroy the majority of an enemy's forces in one strike. It's less than useful in asymmetric warfare (such as fighting the Orks who canonically defeated Boreale) where they simply won't care about your many marines in one localized area.
Hilariously, the Tau notice that last part, and describe this Space Marine tactic as unorganised waste of men that cost Boreale victory.
The real reason it's never used is because Space Marine chapters rarely set up a base, as they normally show up, hit the biggest targets and leave; and when they do, they're working with the local PDF/Imperial Guard, thus do not need Marines to defend the area.
Another drawback to Boreale's defenses is an engine limitation (or perhaps developer oversight): when fighting the Blood Raven stronghold the drop pods that spawn around you only ever contain 4-man fireteams armed only with bolters, hardly a threat to any player-controlled army. This results in the Blood Ravens being one of the easier strongholds to wipe out. Were they 8- or 10-man units with special weapons they'd have a much easier time whittling the player down.
As of the release of Dawn of War 3, the Shteel Rhein maneuver makes a comeback.
Although it isn't mentioned outright in the storyline or from a character, in particular, it can be heard by random chance whenever a player calls in Deathstorm Drop Pods. The Deathstorm Drop Pod is created in the orbital strike slots only, it serves as a defensive weapon to work as area denial and is sent into any location on the map where the player has visibility. When dropped, one of the lines said consists of: "INITIATING STEEL RAIN PROTOCOL". Either making the maneuver canon or a nice nod to Soulstorm.
The difference here is that the Deathstorm is basically a drone turret instead of filled with a half squad of Space Marines, Making it a much smarter implementation compared to the tactic used by Boreale and Guilliman. Elsewhere such as the tabletop in 40k and HH games. It's also a decent troll tactic. Dropping Deathstorms within the shooting range of the objective will force an opponent to waste a turn trying to kill a small number of them.
How it probably should have been used
Steel Rain was likely penned with the idea on a way for how a 13th Hour SM calvary (the Battlebarrge just arrived over the allied fortress) could break an enemy siege. With proper manpower and timing, Steel Rain should break an enemy advancing spear. Unlike Shteel Rhein, It probably was expected to relive another entrenched imperial force(Guard or another chapter) as one of the Chapter's first planetfall action. Probably was not intended for a single chapter to divide their forces to be both defender and Deepstriker. Although, from the structure of Chapters, it strongly seems that Guilliman intended for Chapters to fight together instead of scattering individually deployed companies.
- Drop Pods, the vehicles frequently used in Steel Rain (one might say that they are the Steel Raindrops).
- Steel Rehn, a parody song written to the tune of Tay Zonday's "Chocolate Rain" song, but with lyrics sung by Captain Boreale.
- Katyusha, a legendary WW2 artillery piece.
- Hwacha, the original steel rain.