I know this is probably not a good answer, but I will try.
As you note, V1.1 FT3 is a label. One can only guess (and I did not write this part of the guidance) what the purpose of the label is. It could be a milestone reached during development. Again, I can only speculate that FT3 might mean (based on other contexts
where we have abbreviated Feature Team as FT) that this is a milestone for Feature Team 3. It could be this indicates feature complete for a particular team or feature.
This answer probably raises more questions than answers.
I am not particularly comfortable with the content of the Scenearios Document. It is likely to be completely rewritten for the next release of the Guidance.
I am sorry the narrative doesn't do a better job of explaining the illustration. Or I am sorry the illustration doesn't just stick to the narrative.
Sorry I don't have a better answer. Bottom line, this label is not critical to an understanding of the Basic Branch Plan.
VS ALM Ranger