Simplify the following text for a Grade 10 native English-speaking person.
Original: The COR designation had previously been used for the 6-PUL units and was reused by them during World War II when the Pullman car was stored , but this stock was different from the 4-COR units .
Simplified: The Cor designation had previously been used for the 6Pul units and was reused by them during World War II when the Pullman car was stored , but this stock was different from the 4Cor units .