Hi Juan, can you please post the .3dm model file for your fillet problem?

For doing a fillet, there are different results depending on the selection.

If you select 2 curves that are not edges, you will do a curve fillet between them.

If you select a single curve that has corners (for example a rectangle), you will do a curve fillet on the corners.

If you select edges, those edges will fillet.

If you select surfaces of a joined object, all the edges that belong to that surface will fillet.

If you select an entire object, all its edges will fillet.

The last case if you select 2 surfaces that are separate surfaces not joined to anything else - in this case it will do what is called a surface/surface fillet.

If you can post a model file it would help me undertand what is happening.

- Michael