The Rendering Pipeline is the sequence of steps that are taken by the OpenGL rendering system when the user issues a rendering command. The pipeline is a sequential series of steps, where each step performs some computation and passes its data to the next step for further processing.
The OpenGL specification is very clear on the order in which objects are rendered. Specifically, objects are rendered in the exact order the user provides. While implementations are free to adjust the order internally, the rendered output must be as if it were processed in order.