From the guide: "NAV: Navigation info (0 is the initial point, 1-29 are for FP-A, 30-33 are for PR, 34-39 are for PP, 40 is SPI, 41-49 are for mark point, 50-59 are for airports)."
Not that this is (still) accurate... I've seen ~nothing~ in 40 a number of times... and when there IS a coordinate there, I'm typically not certain how it got there or why.