What determines whether a mechanism is nucleo/electrophillic addition/substitution?

So I recognise what an electrophile and a nucleophile *is*, but not all mechanisms (that I know of at least) have one species substituting or adding. For example, propanone added to NaBH$\_4$ has an initial step of a hydride (nucleophilic addition), and then a
H$^+$ being added to an O$^-$ (electrophilic addition).


I'm a bit wary of this example as the second step doesn't involve a carbon. So, another example would be the addition of HBr to ethene. The first step has the addition of H$^{\delta+}$ (electrophile) and then Br$^{-}$ being added as a nucleophile.


[This website](http://www1.biologie.uni-hamburg.de/b-online/library/newton/Chy251_253/Lectures/EAddition/Addition.html#:%7E:text=The%20first%20step%20involves%20addition,as%20an%20electrophilic%20addition%20reaction.) offers a solution by specifying that the rate determining step takes precedence, but I can't find anything to corroborate this




Any reaction that is either a nucleophilic or an electrophilic addition/substitution is in fact *both* a nucleophilic and an electrophilic addition/substitution. Whatever a nucleophile combines with can also be called an electrophile that attacks the nucleophile; textbooks just adopt a single perspective.


We also have examples where the *same* reagent is both an electrophile and a nucleophile to different parts of the "substrate". [Grignard reactions](https://chemistry.stackexchange.com/a/159755/17175), in which the Grignard reagent has both a nucleophilic carbon atom and an electrophilic magnesium atom, are one such example.


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