This book has the most bizarre numbering system, there are actually at least three, I have ever seen.
In the first section, descendants of the planter, each generation is prefixed by a capital letter.
The section on South Carolina families are indexed with a different letter prefix.
Then there is a section on “unknowns.” There are a lot of these. Each starting person has a U appended to the number of the starting person for that unknown family. That person’s known descendants are numbered in an outline form. The parents of the “U persons” are unknown, thus the connection with the planter, if any, is unknown.