It teaches ONE generation composed of overlapping groups within that generation.
Which either way it still makes no logical sense since the people who lived and witnessed the events of 1914 including C Russell and J Rutherford died and anyone who was born around 1914 have also died, so how can a dead person be included and grouped into a collocation of a greater generation ?
If one were to accept that a single generation is one single human life span of no more than 100 years, once you past a date time such as starting from 1914, that generation can not be included to any following generations.
If you were to do that that would also mean any generations that lived prior to 1914 would also have to be included from an ideological concept.
The WTS came up with a theological spin that has no relevance to intellectual honesty or either scriptural support from the bible.