What does "mission mixing" mean for long-range aircraft? I know very little, like long-range aircraft will be used for short-range and medium-range operations. But what is the purpose of using them like that? Also I want to get complete details of "mission mixing" in layman's terms.
Mission mixing appears to be running computer simulations (or other probabilistic analyses) to assess aircraft engine components' reliability also under non-typical or non-designated use. All for the purpose of design or maintenance/inspection strategies, I would assume.

So, mission mixing is not something you would purposely do with actual aircraft. (Or maybe you would, but then it is not called mission mixing.)

Here are a few articles mentioning it:

https://arc.aiaa.org/doi/pdf/10.2514/1.13142
https://asmedigitalcollection.asme.org/GT/proceedings-abstract/GT2012/44731/439/247656

http://en.cnki.com.cn/Article_en/CJFDTotal-HKXB902.017.htm
https://www.astm.org/DIGITAL_LIBRARY/STP/PAGES/STP14812S.htm