Hanging partitions are partitions that do not create zones because they do not connect at both ends with other walls/partitions. Hanging partitions are created in the same way as ordinary partitions using the Draw partition tool, but are finished by pressing <Enter> (or the End partition stream command from the right click menu) to define the end of a partition stream. They are shown in blue (below).
Hanging partitions are modelled non-geometrically in EnergyPlus as Internal thermal mass.
Also if Automatic block zoning is switched off, all new partitions are treated as outline partitions (shown in blue and do not create zones) until converted to standard partitions by using the Rezone block command from the Tools menu.