I am collaborating on a text which includes a sentence like
This is always true of subset A and, here, it is also true of subset B.
A collaborator has asked if I should write "true for" instead of "true of", and this has led me to question why I decided to write "true of" in the first place.
While I found some discussions on Google, none clearly distinguished to me when to choose one over the other (or even if both are correct).
In one hit, one answer offers the example
There is a difference:
be true (for someone/something): Carleta is from Valencia and the same is true for he [sic] friend María.
be true (of someone/something): It rains a lot in the northwest of England, and that is especially true of Cumbria.
with the explanation
In the first sentence, Carleta and Maria are two separate entities, while, in the second sentence, the northwest of England includes Cumbria, so there is a relationship between these entities.
"Engineers are smart people and the same is true for John" -> John is a smart person just like engineers, but he's not necessarily one.
"Engineers are smart people and that is especially true of John" -> John is an engineer and, as such, a smart person.
According to this logic, I think I should write "true of", but I don't want to take one Google hit as gospel, especially given the lack of any references.
Another discussion on the same site gives no clear answer and digresses into the meaning of "of" in several contexts.
So, when is a quality "true of" something and when is a quality "true for" that thing?