Hi, dear Samsung just renamed the multitasking feature.
1. Open in split screen view is used mainly to use 2 apps together. If you press that option that particular app will open on top half of the screen and you can choose a second app on the bottom of the screen or vice versa. 2. Open in pop-up window is used to open an app in a popup style (Like chat heads in Facebook Messenger) You can use up to 5 apps together in popup windows together. Hope you got the concept.