The phrase covers two quite different builds. A low-threshold shower keeps a shallow curb, usually 40 to 50mm, and is straightforward in almost any bathroom. A true curbless shower brings the shower floor level with the room floor, and that is not a finish decision — it is a question about what is underneath, answered before anything is drawn.
The floor structure decides how low the entry can go. A tiled shower floor needs fall to the drain, conventionally around 2 per cent — a quarter inch per foot. On a curbless build that fall happens *below* the level of the surrounding floor, so the depth has to come from somewhere. Where there are open joists and access from below, the usual answer is to sister and notch, or to drop the sheathing between joists and recess the tray. On a slab, or a condo floor, there is nothing to recess into: the entry stays low-threshold, or the rest of the bathroom floor is built up to meet it. Which applies is visible in about ten minutes on site, and it changes the scope enough to be worth settling before demolition.
Drain choice follows from the same constraint. A centre drain makes the floor fall towards a point from four directions — four planes and a lot of small cuts, awkward under large-format tile, which does not like being bent. A linear drain at the back wall or along the entry lets the whole floor fall one way as a single plane, so a 600mm tile sets flat, and it concentrates the fall where it matters most on a curbless build, which is the entry line.
Without a curb, containment becomes a design problem rather than a given, and waterproofing extends well past the shower footprint for the same reason. The common failure in retrofitted curbless showers is a membrane that stops where the tile pattern changes — an assumption about where water will go, rather than a plan for where it does.