Short definition
A one-dimensional length measurement (a metre measured along a single line) used for skirting, trims, transitions and any flooring product sold by the metre.
A linear metre is exactly one metre measured along a single straight line. The word "linear" is there to distinguish it from a square metre (m²), which measures area in two dimensions.
In a flooring takeoff, linear metres show up for any product priced or installed by the running metre: skirting boards, scotia, quad, transitions and threshold strips, stair nosings, and broadloom carpet bought from a roll where one dimension is fixed (e.g. a 3.66m or 4m wide roll, with run length measured in linear metres).
The most common confusion is between perimeter and linear metres. They are not the same thing. The perimeter of a room is the sum of all wall lengths in linear metres — a 5m × 4m rectangular room has a perimeter of 18 linear metres. But the linear metres of skirting you actually quote will be lower than the perimeter, because you subtract door openings.
AreaSum reports each room's perimeter in linear metres next to its area in m². For doorways and other gaps where skirting is not installed, drop the excluded segments into the room's notes — the perimeter figure stays accurate, and the trim quantity gets a quick manual deduction at quote time.
Worked example
A 5m × 4m lounge has an area of 20 m² and a perimeter of 18 linear metres. If two doorways are 0.9m each, the skirting line in the quote is 18 − 2 × 0.9 = 16.2 linear metres.