The 'dressing station' that Tom referred to was a Regimental Aid Post.
Here is a RAP in open ground near Monchy-le-Preux, 1917

Front-line units, such as infantry battalions, were able to provide only the most superficial medical care. Located near the front line, often in a support or reserve trench, was their Regimental Aid Post, attended by the Battalion Medical Officer and his orderlies and stretcher bearers. A wounded man would either make his own way there if possible, or be carried there. The facilities were crude and often just sufficed to carry out light first aid, give the casualty a drink, or just pass him down the chain to the Advanced Dressing Station. The RAPs were manned by troops of the infantry or other unit.

Casualties moving on to the ADS were moved by hand carriage, wheeled stretchers, trolley lines, etc as conditions permitted. Those wounded men who could walk, did so. From 1916, relay posts for stretcher bearers were established every 1000 yards or so. To avoid congestion, certain communication trenches were allocated for the removal of casualties.

The Field Ambulance attached to the infantry brigade provided one or more Advanced Dressing Stations in reasonable proximity of the front lines. The ADS was better equipped than the RAP but could still only provide limited medical treatment. Men's wounds could be dressed, and some emergency operations carried out. In times of heavy fighting, the ADS would be overwhelmed by the volume of casualties arriving. Often, wounded men had to lie in the open on stretchers for a considerable time. The wounded man would be passed on down the line to a CCS, often by the wagon transport of the Divisional Supply Column. Buses, charabancs, light and broad gauge railways were also used as conditions allowed.

In addition to the ADS, the Field Ambulance was also responsible for the stretcher bearer relay posts, walking wounded collecting stations, sick collecting stations and rest stations. A larger version of the ADS, the Main Dressing Station, was often provided for the Division.

The CCS was the first large, well-equipped medical facility that the wounded man would visit. It's role was to retain all serious cases that were unfit for further travel, treat and return slight cases, and evacuate all others. It was usually a tented camp, although in the static trench areas the accommodation would sometimes be huts. CCS's were often grouped into clusters of two or three in a small area, usually a few miles behind the lines and on a railway line. A typical CCS could hold 1,000 casualties at any time, and each would admit 15-300 cases, in rotation. At peak times of battle, even the CCS's were overflowing. Serious operations, such as limb amputations, were carried out here. Some CCSs' were specialist units, for nervous disorders, skin diseases, infectious diseases, certain types of wounds, etc. CCS's did not move location very often, and the transport infrastructure of railways usually dictated their location. Most casualties came away by rail, although motor ambulances and canal barges also carried casualties to Base Hospitals, or directly to a port of embarkation if the man had been identified as a Blighty case. (In 1916, 734,000 wounded men were evacuated from CCS's by train; another 17,000 by barge, on the Western Front alone. There were 4 ambulance trains in 1914, and 28 by July 1916). The serious nature of many wounds defied the medical facilities and skills of a CCS, and many CCS positions are today marked by large military cemeteries.

CCS's also catered for sick men. Generally, considering the conditions, the troops were kept in good health. Great care was taken in reporting sickness and infection, and early preventive measures were taken. The largest percentage of sick men were venereal disease cases at 18.1 per 1000 casualties; trench foot was next with 12.7.

Until mid 1915, the CCS was known as a Clearing Hospital. Generally there was one provided for each Division.

From the CCS, the casualty would be evacuated to a hospital.

Once admitted to a Hospital, the Tommy stood a reasonable chance of survival. More than half were evacuated to the UK from a General or Stationary Hospital for further treatment or convalescence.

The Stationary Hospitals, two per Division, could hold 400 casualties. The General Hospital could hold 1040 patients. They were located near the Army's principal bases at Boulogne, Le Havre, Rouen, Le Touquet and Etaples. The establishment of a General Hospital included 32 Medical Officers of the RAMC, 3 Chaplains, 73 female Nurses and 206 RAMC troops acting as orderlies, etc. The hospitals were enlarged in 1917, to as many as 2,500 beds.