Mixed Preposition Definition ?

[awons]

I believe definitions can help me greatly to understand a concept.
Tried my best searching for a definition for "Mixed Preposition". Everywhere i looked, i found only exercises for the same.
Pls help defining "Mixed Preposition"
Thanks in advance
Awon

 

[Tdol]

To me, it just means that the exercise could contain any preposition, rather than exercises restricted to related ones like in/at.