In your example, the two are the same. But the issues revolve around pointers.
volatile uint8_t *foo; tells the compiler the memory being pointed to is volatile. If you want to mark the pointer itself as volatile, you would need to do
uint8_t * volatile foo;
And that is where you get to the main differences between marking the struct as volatile vs marking individual fields. If you had:
volatile foo f;
That would act like:
uint8_t * volatile field;
and not like:
volatile uint8_t *field;
if you declare a structure with volatile then all its members will also be volatile
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