When is it better to use non-concrete types in structs?

[Tamas_Papp]

combinatorial explosition of types is mentioned in the context of

https://docs.julialang.org/en/v1/manual/performance-tips/#The-dangers-of-abusing-multiple-dispatch-(aka,-more-on-types-with-values-as-parameters)-1

but I agree that some additional notes about it would be useful.