Group 1 - The article discusses a recent incident where a user received a notification on the 12306 App regarding the availability of a specific first-class seat located behind the business class, which has certain limitations [2][4] - The customer service of 12306 clarified that the notification was added due to the unique nature of some first-class seats being located in the business class section, which may not meet the usual comfort expectations [4] - There have been complaints from users who purchased first-class tickets only to find themselves in seats that differ significantly from standard first-class seats, leading to discomfort during travel [4] Group 2 - The customer service explained that the mixing of business and first-class seats occurs due to train composition limitations, where not enough business seats can be allocated, resulting in a "mixed seating" arrangement [4]