These are the three biological instincts (SP - self-preservation, SO - social, SX - sexual) that impact specifically how exactly a person's number shows up.
One of these is likely the DOMINANT instinct, or the one that gets unconsciously relied on so much so that it may show up MORE than is appropriate (therefore needs to be reined back). Another is likely to be the REPRESSED instinct that gets unconsciously bumped back into the person's blindspot so much so that it doesn't show up as often as it needs to (therefore needs to be sought after and developed like an underused muscle) to balance the person out.
There are three different "versions" of each Enneagram number depending on which of the three is the dominant instinct, i.e., there are 3 versions of fours (4SP, 4SO, and 4SX) and all the other numbers.
As a result, there are 9 types in the Enneagram, but 27 SUBtypes (9 types x 3 versions each) and 54 SEQUENCES (27 x 2 if you also consider which of the remaining non-dominant instincts is REPRESSED).
For example, I am a 4 with a dominant social instinct and a repressed sexual instinct, so my TYPE is a 4, my SUBTYPE is
a 4SO ("Social 4"), and my SEQUENCE is 4 SO-SP-SX.
There's a whole lot more to be learned about the subtypes, but it makes the Enneagram so much more specific and easier to find practical applications for, especially because for Fours and Sixes, their subtypes look SO DIFFERENT from each other that the subtypes may as well be different numbers altogether.
For more information, look up Beatrice Chestnut's books (The Complete Enneagram, The 9 Types of Leadership, The 27 Subtypes): https://beatricechestnut.com/products-services/
Here is her blog: https://beatricechestnut.com/2017/08/instinctual-subtype-dimension-enneagram/