Central systems condition a room from several registers. A ductless head does it from one point, which makes its position a design decision rather than a convenience.
The rules that matter
Throw down the long axis. The head should blow along the length of the room, not across its width into a nearby wall.
High on the wall, typically within a foot of the ceiling, with the manufacturer's clearance above it — the unit draws its return air in through the top and it will not do so if it is jammed against the ceiling.
Not directly over a bed or a desk. In cooling the air arrives cold and moving, and people sitting under it are uncomfortable however well the system performs.
Not blowing straight into a wall or a large piece of furniture a few feet away.
Not above a heat source such as a radiator or a lamp, which distorts the unit's own temperature sensing.
The sensor problem
Most ductless heads read room temperature at the unit, high on the wall.
Warm air collects near the ceiling, so the unit sees a warmer room than the occupants do in cooling, and in heating it sees a warmer room too — which is why some ductless installations overheat in cooling and undershoot in heating.
Many models offer a follow-me or remote sensor mode, which uses the handset's own sensor at occupant level. It is a setting worth enabling and it is frequently left off.
Heating changes the ideal position
Warm air rises, so a head mounted high has to push heat down against buoyancy. In heating-dominated use this is a real limitation.
Floor-mounted heads are the answer where heating is the main job. They sit low on the wall, deliver warm air at floor level, and heat a room far more evenly. They cost about the same and are much less commonly proposed.
In a cold climate where the ductless system is the primary heat, this is worth raising specifically.
Open plan
A single head at one end of a long open space will not condition the far end well, whatever its capacity.
Two smaller heads at opposite ends work better and modulate lower, which improves the humidity behaviour as well. This is one of the few cases where more heads is straightforwardly the better design rather than an upsell.
Ceiling cassettes
A cassette recessed into a ceiling distributes in four directions and suits a square room. It disappears visually, which many people prefer.
It needs a ceiling void, structural support, and access for service, and it costs more to install. In a room with an accessible ceiling it is often the best answer.
The practical constraints
The line route. Every head needs its refrigerant lines, condensate and cable run to the outdoor unit. A head on an interior wall means a concealed run through the structure, which is much more work than a head on an exterior wall.
The condensate fall. Gravity drainage needs a consistent slope, and a head placed where that is impossible needs a pump.
Where the outdoor unit can go, which constrains the practical head positions on the far side of the house.
What to do before agreeing a position
Stand in the room, imagine the airflow along its length, and check where people actually sit or sleep.
The installer will propose the position that is easiest to pipe. That is a legitimate consideration and it is not the only one, and the conversation is much easier before the hole is drilled.