Fill in part of segment without adding whole model

Hey all, I cannot seem to figure out how to get this section of my model to fill in (i.e: make the segment whole) without adding back the entire model. I have just figured out how the “masking” function works, and am still confused about the inter-workings of it all. Any and all help is appreciated, especially if you’re willing to dumb it down for me haha

If you are working directly on a surface mesh, you can use the SurfaceToolbox module, and navigate to the Fill Holes function…