Stacking skill: the possible stacker meaning
The guides describe a skill called Stacking: more of the same item in one slot. Stacker may refer to that skill, a mod, or player shorthand. The sources reviewed do not establish which meaning the search intended.
Ranks, not an item
You still unlock Stacking the way you unlock other skills: a point and the leaflet, lower rank first. It does not replace Backpacker, the skill described as one extra bag slot. Stacking changes how many copies sit in a slot you already have. Backpacker changes how many slots you carry.
Why the search says stacker
The keyword research records stacker 1, but does not prove why people use that wording. This page covers Stacking because Raintek documents it. Nexus inventory mods are a separate category; their names are not evidence that stacker is an official skill or item name.
I do not have a list of which items become stackable at rank 3. Raintek says the third rank widens the set. The item names themselves are not in the excerpt I used.
Questions
What do the three ranks do?
Raintek's guide sets the stack cap at 2, then 3, then 4. The third rank also widens which items may stack. The community table phrases the same idea as maximum stack size plus one per rank.
Is Extra Slots the same skill?
No. Extra Slots is a Nexus mod that adds bag space past the normal cap. It is not a rank you buy with a leaflet.