Short definition
The visible join where two pieces of broadloom carpet meet — usually placed behind a door or against a low-traffic wall to keep it out of sight.
A carpet seam is the line where two pieces of broadloom carpet are joined together. Seams are unavoidable on any room wider than the roll — a 5 m wide living room on a 3.66 m broadloom roll must take at least one seam, full stop. The estimator's job is not to avoid seams but to place them where they will not be seen.
The convention installers follow: put the seam in a doorway or against a low-traffic wall, and run it parallel to the longest sight line of the room. A doorway join hides the seam behind the door swing for half its visible life and is the default placement when a low-traffic wall is not available. Joins running across a sight line are visible from every angle; joins running with the sight line vanish against the pile direction.
In a flooring takeoff, seam location affects two things: total linear metres ordered off the roll (a poorly placed seam can force an extra drop), and the offcut profile of the job (a seam that lines up with a room boundary leaves a cleaner offcut than one that ends in mid-room). Both of those land in the wastage allowance, but the underlying decision is geometric.
AreaSum reports the raw room dimensions and lets the seam placement decision live with the installer. The carpet cut list calculator at /calculators/carpet-cut-list flags rooms that need joins so the seam placement question gets attention before the carpet is cut.
Quote from a flooring estimator
"We try not to have joins where possible. If we have to have a join, we will always try and put it in a doorway or out of sight. Ninety-nine per cent of the time it is always a doorway join."