The building has adjoining or attached building(s) on one side; this includes corner buildings (end buildings within a block) and semi-detached buildings (with two housing units).
The building shown in black in this plan view has an adjoining building on one side.
An example of a building within a block with adjoining buildings on one side, Vancouver, Canada (left photo: S. Brzev, right: Map data ©2013 Google, Province of British Columbia, DigitalGlobe)
Semi-detached townhouse, adjoining building on one side, New Zealand (L. Allen).
Semi-detached house, adjoining building on one side, Constitution, Chile (S. Brzev).
A building on a corner, with an adjoining building on one side, Seattle, USA (S. Brzev).