I wonder whether it is
- Functional/Geometric Flexibility (engine type) or
- Functional/Geometric Adaptability (-"-)
What's the difference? What's the right choice in this context?
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Adaptability is the primary meaning in this context. Flexibility describing this property is a kind of metaphor, bendable = possible to adapt, with the basic meaning of a physical property, bends and returns to original shape.
In your case, to me at least, it feels like "Geometric flexibility" may be somewhat ambiguous, possibly meaning floppy wiggly springy stuff. When no structural ambiguity exists (nobody expects "flexible schedule" to mean "unrolling on its own when taken out of the box") they are quite synonymous, but "flexible geometry", "flexible engine" - these may mean "made of springy plastic" too.