The Seal Unit Assembly (sometimes used with a Locator) forms the bottom components of the tubing string. The Packer Seal Bore / Seal Bore Extension and the Seal Unit Assembly worked as matching pair for connecting of the upper string to the lower string. This allows the retrieval of the upper tubing string for repair without pullling the Packer and the lower string.
The Seal Unit Assembly is typically longer than the sealbore to enable some movement of the components while maintaining an efficient seal.
