Everything is very simple.
By choosing between many different Individual Training Focuses for your player you can determine which of his attributes should have higher chance to increase compare to other his attributes and it’s simple as that.
When I suggest choosing “Trequartista” Individual Training Focus for STCL/STC/STCR positions in this tactic then it means that I find that “Trequartista” Individual Training Focus trains the best possible combination of attributes for STCL/STC/STCR positions in this tactic.
SI gave every Individual Training Focus names that are similar with the roles in order to help beginner players because when you set Individual Training Focus for some of your players that are similar with his role then most of time you won’t be too wrong about it but setting the individual training focuses for all your players according to their roles doesn’t mean that you do the most efficient thing possible, for example, there are a lot of default tactics in the game but I’m sure that no one thinks that these tactics is the best what you can get and the same logic with the Individual Training Focuses.
It’s important to understand that it’s only you who knows what players do in your tactics because the behavior of every default role can be changed drastically by adding PIs and TIs, for example, two tactics have CM position that has Central Midfielder role but it possible to make that this CM position played very different in each tactic so it means that for CM position in each tactic you need different type of players with defferent attributes even they've got the same role in the tactics.
Also, it’s important to understand that it would be very naïve to expect that SI just tell you which attributes are “overpowered” and the most useful in the current FM version. Only by playing the game you can find out it, for example, you might find out that if your striker has “16+” Composure then it works like magic, he scores 100% of his chances from any distance so knowing that you might want to start training Composure attributes of all your strikers at the first priority.
