Tail lines are used to track the inheritance of mitochondrial DNA and Y chromosomes.
Since mitochondrial DNA is inherited Maternally the Female Tail line is determined by tracing your dog back along its pedigree through the maternal line. So basically following the very bottom of the pedigree all the way back until you get to Unknown African Dog.
All of my girls, Rally, Rio, and Sophie have the same female tail line since Rally is the mother of Rio and Sophie. My girls female tail line goes back to Bokoto of Blean. TC the girl that I have bred Nicky to has a female tail going back to Amatangazig.
The male tail line traces the inheritance of the Y chromosome since it is only inherited from father to son. You can trace the male tail line following the very top of the pedigree back to Unknown African Dog.
Nicky's male tail goes back Wau which I think is one of the most common male tail lines in the gene pool. Cole, Rio's littermate, has a male tail back to Kindu. Zeke, Sophie's littermate, has a male tail back to Avongara Diagba.
Some of the founders no longer have tail lines meaning their Y chromosome if male and mitochondrial DNA if female has been lost from our gene pool. Some of these dogs still have descendants just not tail lines, some founders we have lost all together.