Spam is a term refers to the tactics of overwhelming the enemy through superior number and scale. Due to spam-addicted commanders may use little, if any, tactics or micromanagement, and enormous casualties as often outcome, this kind of tactics is always criticized for lack of skill required and is usually viewed negatively and seen as signs of poor skill and game design. There are four categories of spam in the Command & Conquer series: Unit Spam, Defense Structure Spam, Garrison Spam and Production Structure Spam.
All the non-epic units can be the candidates for spamming identically or compositely. Field commanders tend to mass-produce "front-line" units of low-tier or medium-tier, since they're balanced on both offense and defense, and relatively not demanding on both building cost and building time. By these reasons, basic infantry and MBTs are the most common units to be spammed. However this is not absolute, units with one particular distinguishing aspect, are also frequently spammed. Such as Apocalypse Tank, its sheer might is hard to be overlooked, despite its high-tier and expensive cost, slow production and movement.
Air units spam is common as well. An example are the Venoms, since they can attack both air and ground targets, and more importantly, the little dependance of replenishment. After upgrading them with laser capacitors, Charged Particle Beams or Supercharged Particle Beams, a group of them, with enough number, can easily eliminate almost anything they come across.
Artillery units like the V4 Launcher and the Dreadnought are also spammed by some field commanders to conquer enemy bases with tough defenses. However, artillery spam may inflict excessive damage to both enemies and civilians. So careful micromanagement is needed if commanders want to minimize neutral casualties or to seize enemy assets.
Defense Structure Spam
Defense Structure Spam is used for both defense and offense. Constructing many defense structures to form a huge and deep firepower grid can be quite challenging to enemy strike forces. Some commanders also set defense structure grids very close to enemy bases, isolating them from other sectors on the battlefield and destroying anyone attempting to leave bases (e.g. harvesters and aircraft). Moreover, artillery squads can fire in safety behind defense lines, commanders even can build production structures like war factories to assemble strike forces directly under enemies' very noses.
In games before Red Alert 2, there was a variant called "Superweapon Spam". The commander could build many superweapon structures to shorten recharge time and/or avoid their superweapons go offline since these buildings are "attractive" to enemies. But since Red Alert 2, each superweapon is only allowed to be built once at the same time by default, for balancing reasons due to their tremendously increased destructive powers.
Garrison Spam is mostly used in urban operations as a combination of unit spam and defense structure spam. The tactic is garrisoning every building can be garrisoned and converting the entire battlefield into an enormous firepower grid. This first appeared in Red Alert 2, taking advantage of its new civilian building garrisoning system. However, in Red Alert 2 and Yuri's Revenge, only basic infantry units can station a garrisonable structure, but their en masse firepower, with supports from engineers, additional units and defense structures, making this tactic useful to slow-paced commanders.
Bunkers enable this kind of spam even if there are no civilian structures to be garrisoned.
In following games, to avoid abuse of such spam, anti-garrison units and support powers are introduced. In Tiberium Wars, Command & Conquer: Red Alert 3 and their expansion packs, civilian structures are no longer repairable, reducing the effectiveness of Garrison Spam greatly. However in Tiberian Twilight, civilian structures can self-reconstruct if destroyed and cannot be cleared of garrisons.
Production Structure Spam
The previous three kinds of spam consume resources severely and may require multiple production structures like Construction Yards, Barracks, War Factories, etc. To collect and store more resources, dozens of refineries and silos are needed. To feed all above, power plants must be spammed. So a spam-addicted commander has to spam production structures before or during the progress of swarming anything else.
Interestingly, the initial reason of spamming production structures is because from Tiberian Dawn to Yuri's Revenge, there is only one production line for each category of products, which means field commanders can only generate one structure, one infantry, one ground vehicle, one aircraft and one naval vessel, at the same time. In Red Alert 2 and its expansion pack, although production structures and defense structures are separated, but still suffer "single production line problem" for each. So it is imperative to set up multiple Construction Yards, Barracks, War Factories to accelerate manufacture progress.
Airforce-centric commanders may spam Helipads, Airforce Command Headquarters, Airfields and other similar structures to quickly rearm and repair large quantity of airborne units. Especially in Generals, Tiberium Wars, Red Alert 2, Red Alert 3 and their expansion packs, for in those games air units only can land in "empty slots" of those structures.
Spam in Command & Conquer 4: Tiberian Twilight
In Tiberian Twilight, Crawlers, Command Points, Power Limit and rare civilan buildings prevent spam. However, since many critics claim spam is one of the most important signature of Command & Conquer series, some trainers have been used to re-enable spam in Tiberian Twilight.