Attack Move Command
The Attack Move command, also known as Fight, tells units to proceed to the command's designated goal while engaging encountered targets.
Units with their Unit AI state will use their tactical AI when encountering the enemy - raiders will swarm skirmishers, skirmishers will kite assaults, and so on. This is very distinct from e.g. Move command, which doesn't engage the AI and can be considered a manual override.
Builders given the Attack Move command will indiscriminately use their buildpower to perform every possible build task en route, such as reclaiming any wrecks and other features, repairing damaged units, and assisting incomplete constructions.
For this reason, a factory whose rally point is given using the Attack Move command will usually have every builder produced at that factory automatically assist the factory. This can be both beneficial or harmful - on one hand, it automates construction of new buildpower for the factory; on the other, this means the builders will not go where they may be more needed unless explicitly ordered.