An internal fluid is characterised by its mass density, internal pressure (above hydrostatic) and a level above the mudline. These are specified via a Properties dialog on the Internal Fluid component.
Add Line Dialog
You will note the drop-down list for Internal Fluid near the bottom of the dialog, this allows you to choose one of the Internal Fluid component currently defined in the project. The specification of internal fluid is of course optional; pipelines are empty (air-filled) by default. If you associate an internal fluid with a Line component, the whole line is assumed to be flooded, and the internal fluid is associated with all pipe sections within the line.