Ports

Each row of a source, transformation, or target definition is called a port. A port represents a single column of data in a table. There are four types of ports:

Input ports receive data
Output ports provide data
Input/output ports are ports that data passes through
Variable ports store components of expressions

Sources provide data, so they have only output ports.

Targets receive data, so they have only input ports.

Transformations have input/output ports if data passes through them untouched; otherwise, they may have either an input port or an output port, depending on how they modify the data.