The way I understand it - the seal is for the design and the construction documents. Supposedly, whenever there is a significant change - the engineer is supposed to issue new drawings or sketches that are also sealed. That doesn't always happen.
In my experience, the contractor sends his mark ups to the consulting engineer and they make the changes and issue the "as built" drawings to the owner.
It does depend on how the project is set up. We typically only do as builts for state funded, i.e. universities, government buildings, etc.
If we are not responsible for the as builts, we will send the autocad file to the contractor, but without the seal, title block and other company information. The drawings then can be put on the contractor's title block and issued that way.