Here is the description of SINK and SOURCE from the IfcFlowDirectionEnum
SOURCE: A flow source, where a substance flows out of the connection.
SINK: A flow sink, where a substance flows into the connection.
But in the following picture (1), (2a), (2b) and (3) a substance would flow out of the connection (depending from which side you see it):
Is (1) and (2b) a SOURCE and (2a) and (3) a SINK?