Inserter capacity bonus (research)
When inserters are moving items from one stack to another (or loading/unloading trains) they can move more items at the same time after researching Inserter item stack size bonus 1. It works only when moving from stack to stack. This means if either the target, or the source has no stack, the stack size bonus doesn't work. Therefore, the bonus typically only applies when moving from machine/chest to machine/chest, and not from machine/chest to belt or belt to machine/chest.
A slow Inserter with stack size bonus of 4, which moves from chest to chest, can easily keep up with a Fast inserter, moving from chest to a belt. This is why it is recommended to directly move items that are required in high quantities at high speeds during automatic crafting.
A common beginner error is for example, to use always belts for transporting Copper cables. This should be avoided: one item copper plates is assembled to two items of cable! Try to built copper cable assembly directly besides the target assemblies and fill with inserter from assembly to assembly.
Another problem is, that with inserter stack size bonus it can break some constructions with the Circuit network: it is not guaranteed, that some circuit condition, which works with stack size 1 is still working with stack size two. This is especially true for the equal ("=") condition.
This bonus is is also often called "inserter bonus", "stacksize bonus" , "stack bonus", etc.
It is directly related to Logistic robot cargo.
|Inserter item stack size bonus 1
|Inserter item stack size bonus 2
|Inserter item stack size bonus 3
|Inserter item stack size bonus 4