Definition
The iso is an offensive cutter who is deliberately isolated from the rest of the formation — typically alone in the central or open-side space — so that they can attack their defender in a 1v1 with the rest of the cutting lane clear. The iso role exists to remove ambiguity: there is one cutter the handlers are looking for, and the structure ensures the lane is open for them to work in.
In Context
In our Spread Stack, the iso is the central or open-side cutter who works alone while the Side Stack holds the sideline. Their job is to read the Mark and the Force, and to threaten both an under and an away cut in sequence — see 4 Lines - Away and Under for the patterned version.
The iso can also be a handler-iso — a single handler set apart in dump space who runs Give-and-Go sequences with the Active Handler without help cluttering the picture. The principle is the same in both cases: clear the surrounding space so the 1v1 is real.
The iso depends on the rest of the team to not show up. The moment a second cutter drifts into the iso's lane, it is no longer an iso — it is congestion. See Clear the Middle - Quick Ref for the spacing discipline that protects iso play.