Federico Alzamora Previtali

Your understanding of the Geometry description is correct. Fluent will attempt to assign boundary types automatically based on boundary names and their topology relative to the rest of the domain.

You can try providing descriptive names to the relevant boundaries prior to bringing into Fluent Meshing. Fluent will recognize words like "internal", "wall", "inlet", for example.

You can also change the boundary type in the Update Boundaries taks.