Is "formal schismatic" a useful category in practice? Wikipedia says that formal schismatics are those who:


  knowing the true nature of the Church, have personally and deliberately committed the sin of schism.


But if formal schismatics have to truly know the true nature of the Church, is it ever actually a category that can apply to people?

For example, Protestants reject the authority of the Catholic Church and the Pope, and so would not be said, I would think, to know the true nature of the Church.

Likewise, are the SSPX truly formal schismatics if, in their rejection of Vatican II, they believe the true nature of the church is other than that of the Catholic Church after Vatican II?

If you have to know and truly believe in the true nature of the Church in order for your rejection of it to be "formal", then it seems to be that this is a largely academic category, and that there would be exceedingly few actual cases of formal schismatics.
Your Wikipedia article explains this:

Roman Catholic theology considers formal schismatics to be outside the Church, understanding by &quot;formal schismatics&quot; &quot;persons who, knowing the true nature of the Church, have personally and deliberately committed the sin of schism&quot;.[8] The situation, for instance, of those who have been brought up from childhood within a group not in full communion with Rome, but who have orthodox faith, is different: these are considered to be imperfectly, though not fully, members of the Church.

It's not that someone knows the Catholic Church is the true Church, but rather that someone knows the true nature of the Church.  For example, if a Catholic refuses to submit to the Pontiff without understanding the Pontiff's role in the Church, he is not a formal schismatic.  If he does understand the Pontiff's role and still refuses to submit, then he is a formal schismatic.  Only in the latter case does he understand the true nature of the Church.
The OP asks, &quot;What's an example of someone who is a formal schismatic?&quot;
See Canon 751 for the definition of schism.  Schism occurs in relation to a specific Church.  If a Catholic leaves the Catholic Church and joins the Ukrainian Orthodox Church, he would be in schism from the first but not the second.  Two simple examples of formal schismatics would be Luther and Calvin:

Thus the schism was well consummated by [Luther and Calvin] before it was solemnly established by the authority which they rejected and transformed by that authority into a just penal sanction.
-Schism, Catholic Encyclopedia