They are known as sdB stars because their spectra are dominated by hydrogen lines. They are analogous to extremely blue stars in globular clusters known as horizontal branch stars. As such, they represent the next stage after a low mass star has completed its evolution as a red giant. The main problem they pose is how the outer layers of hydrogen have been expelled. Normally, a red giant will switch on core helium burning without losingthe layers of hydrogen around the helium core.
Most sdB stars are extremely hydrogen-rich.
In contrast, the sdO stars tend to be more luminous and hotter, showing principally lines of ionized helium in their spectra. The majority represent the final stage in the evolution of a star before it becomes a white dwarfs. The central stars of most planetary nebulae are, in fact, sdO stars. The majority have electron-degenerate cores consisting of carbon and oxygen, surrounded by oniion layer shells of helium and hydrogen. Both of these may very thin, or even absent leading to some quite unusual surface abundance characteristics amongst the hot subdwarfs.