Hindu calendar only includes Umbra as an eclipse. Penumbra (here B) can affect a wider area of place and last longer as well. Partial eclipse also occurs in the Penumbra. So why is only Umbra counted as an eclipse?
I believe the answer lies in your question itself - Total Eclipse occurs only for an observer in the Umbra region. Penumbral Lunar Eclipses are not visible to the naked eye hence no rituals related to Chandra Grahan are generally observed.
However, if the eclipse is visible then it may be counted. Last night's (Shravan Purnima - 7th August 2017) partial Lunar Eclipse was mentioned even resulted in the Muhurat for Rakshabandhan being shortened.
It is due to the Lunar Eclipse that the time was shortened: