The part is the plastic product you are modeling. You should optimize the modeling of the part, before you model the mold and the feed and cooling systems.
When you have selected the gate location and appropriate processing conditions, select a material; set the injection location; select a Fil analysis; and start the analysis.
Before determining the optimal solution, you may be required to run analyses that use several variations or combinations of process settings, injection locations, cooling configurations, and so forth. You can save copies of a study at various points during the analysis to help you manage this process more efficiently. These copies can be used as baseline data to test different settings and variations of the same model.