The World War I term given to triage areas just back from the front.
The following is taken verbatim from my Lijssenthoek writeup, and explains the reasons casualty clearing stations were required:
- Speed. To maximise the survival chances of the men, they had to be treated as promptly as possible. The medical facilities at the headquarters, located a few miles back from the front, were too far away to provide emergency treatment.
- Versatility. The headquarter's hospitals were too inflexible. What if there was heavy fighting elsewhere on the front and medical services had to be moved to bolster the treatment of soldiers there? What if the enemy was gaining the upper hand, and the front was edging backwards? Supplies and invalids would have to be moved backwards too.
- Simplicity. At the front, quick and efficient sorting of casualties and first aid was all that was needed: patch up the men so that they would survive the journey to a proper hospital. When at the hospital, more advanced techniques, such as amputations and plastic surgery, as pioneered by Sir Harold Gillies, could be applied. The advanced medical services located back from the front were not required here, and would only reduce patient throughput.
For these reasons, a flexible and efficient casualty sorting area was required as close to the front as possible, just out of range of the German howitzers.