Trelis usually relies on the ACIS solid modeling kernel for geometry representation; there is also mesh-based geometry. Geometry is imported or created within Trelis. Geometry is created bottom-up or through primitives. Trelis imports ACIS SAT files. Trelis can also read STEP, IGES, and FASTQ files and convert them to the ACIS kernel. As of version 15.0, Trelis supports direct translators for Parasolid, SolidWorks, and Pro/Engineer for an additional license fee.

Once in Trelis, an ACIS model is modified through booleans. Without changing the geometric definition of the model, the topology of the model may be changed using virtual geometry. For example, virtual geometry can be used to composite two surfaces together, erasing the curve dividing them.

Sometimes, an ACIS model is poorly defined. This often happens with translated models. The model can be healed inside Trelis.