The line set is the pair of insulated copper tubes that carries refrigerant between a mini-split's outdoor condenser and its indoor head: a small liquid line (commonly 1/4 inch) and a larger suction or gas line (3/8 to 1/2 inch, depending on the unit's capacity). It runs alongside the control wiring and a condensate drain, usually through a 3-inch wall penetration hidden under a line-set cover.
Length and lift are limited, and the limits are specific to the model. A typical single-zone system allows a total run on the order of 65 to 100 feet with a maximum height difference between units of 40 to 50 feet; multi-zone condensers set both a per-head limit and a combined-total limit. Manufacturers ship single-zone outdoor units pre-charged for a set distance — often about 25 feet — and beyond that the installer must weigh in additional refrigerant per the install manual. Skipping that top-up, or simply coiling the excess tubing, quietly costs capacity.
This is where install quality shows. The flared connections must be cut and torqued correctly or they leak; the lines must be pulled into a deep vacuum (evacuated) to remove moisture and air before the charge is released; and the final charge must match the actual line length. A sloppy flare or a wrong charge is the single most common reason a new mini-split underperforms or voids its warranty — which is why EPA-certified, manufacturer-authorized installation is worth verifying.