A Data Transformation function consists of several building blocks one needs to know about them to understand how the API works.
Step
A Step is a block of code which performs complex processing logic. It adheres to the Yellowfin Transformation Step API(link to javadoc), as a result of which they can be connected to other unrelated blocks of code.
All steps implement the ETLStep Java interface.
An Inline Step (Field Transformation Step) is a special type, which is applied to a single field and is not visible on the canvas. Inline Steps implement the ETLInlineStep interface.
Flow
A Flow is the link which joins two steps. A Step may have one or more Inputs and Outputs. Flows are identified using Input/Output FlowUUIDs.