The function of the seal 144000 are to follow Christ wherever he goes. These are sealed after the 7 year tribulation period. How would one know this? Because of Rev. Chapter 6 and 7. These chapters are the ones that the following chapters up to Rev. 21 : 5 describe in detail. For instance, like at the end of Rev. 7 when it says God shall wipe away all tears from their eyes. When and where does this take place? Rev. 21:4 When New Jerusalem comes down from heaven. Towards the middle of chapter 7 when one of the elders told John that a great multitude that could not be numbered that these are they which come out of great tribulation. What is this? Rev. 19:4-9 This is the marriage supper of the Lamb. At the end of the 6th seal in Rev. 6 People where praying for the rocks and mountains to fall on them for the great day of his wrath has come. When is this ? In Rev. 16 the 7th vial, this is the destruction of Mystery Babylon in which Rev. chapter 17 and 18 describe in detail. The 144000 are sealed after the destruction of Mystery Babylon. That is why a Lamb is on Mt. Zion in Rev 14 redeeming the 144000 from the earth then the angel announcement that Babylon is fallen is fallen. Note that Babylon is NOT falling it had already fallen before the 144000 were sealed like in Rev chapters 6 and 7.