This is the default mapping mode. Mapping will depend on schema mapping in Project options.
In Automatic mode you are able to deselect certain columns and turn on the Load tables into memory option only. Changing matching table or column in target database will automatically trigger the Manual mode. Learn about manual mapping mode on a Manual mapping page.
Load tables into memory and sort them before comparing slows down the comparison and hugely impacts memory use. Use this only when source and target tables are sorted differently by source and target database server.
You can deselect columns without enforcing manual mapping. Use Deselect all and Select all buttons to quickly change selection on all columns and use your mouse to click on individual selection check boxes from there.