It'll be used as transport to replace ISS crew members.

它将被用于代替国际空间站机组人员的运输工具。

It has been up there for so long that the astronauts are flown up to the space station, where they replace other crew members and carry on their respective duties as part of their assigned mission.

它已经在那里待了很长时间，以至于宇航员被空运到空间站，在那里他们取代其他机组人员并作为其分配任务的一部分履行各自的职责。