A TXV has a sensing bulb clamped to the suction line where it leaves the evaporator. The bulb holds a charge that presses on a diaphragm as it warms. If the evaporator is starved and the suction gas leaves too warm, the valve opens wider; if the evaporator is overfeeding, the gas leaves cooler and the valve closes down.
Unlike a capillary tube, a TXV adapts to changing loads and ambient conditions, so it keeps the evaporator well used across a wider range. It is common on larger reach-ins, display cases, remote systems and cold rooms. Systems with a TXV usually include a liquid receiver and are less sensitive to the exact refrigerant charge.
A TXV must be installed and set correctly. The bulb has to be firmly clamped and insulated, and superheat checked at commissioning. A loose bulb or a wrong setting can starve the coil, so the cabinet runs warm, or overfeed it, risking liquid returning to the compressor. Adjustment is a technician's job, done with gauges and a thermometer, not by feel.