In Streptocarpus the older classification of Hilliard & Burtt (1971) dividing the genus into two subgenera, Streptocarpella and Streptocarpus, was based on growth patterns, with the members of the subgenus Streptocarpella being herbaceous and the subgenus Streptocarpus being either rosulate or unifoliate, often monocarpic, herbs. In the context of the members of the genus known from the African continent and prior to the addition of Saintpaulia into genus Streptocarpus, this made sense and was followed for many years. However, the discovery of rosulate Saintpaulia-like Madagascan species started blurring these boundaries. Already in 1971, Hilliard and Burtt wrote that they thought Saintpaulia rather belonged in the solely African genus Streptocarpus.
Saintpaulia and the Madagascan Saintpualia-like Streptocarpus have seeds with warty or bumpy surfaces, which are called verruculose seeds, which indicated that Saintpaulia probably belonged in subgenus Streptocarpella. Members of the subgenus Streptocarpus have seeds that are netted, which are called reticulate seeds, and none of the Saintpaulia species have reticulate seeds. It is often thought that the DNA data generated by Moeller and Cronk in 1999 was responsible for placing Saintpaulia into Streptocarpus, but this was definitely not the first inkling that the genus Saintpaulia, belonged in Streptocarpus; the seed and other morphological characters examined long before already gave an indication of this.
However, Saintpaulia is historically well known in culture and all species are easily identifiable as members of that group. It is sensible to achieve clarity by treating Saintpaulia as a separate entity within the horticultural context. In my sectional overview of the genus Streptocarpus I will therefore not go into much detail on section Saintpaulia and rather refer the reader to the excellent article on this website.