In a superseded IEG script, subscripts could be added at the top-level (just under the script) or under a page. In an IEG script, a subscript can be added at the top-level or just under a section. This is why there is no one-to-one mapping between old and new subscripts. Top-level subscripts will be migrated as independent scripts (but they won't have any section) and referenced in the scripts, whereas other subscripts will be merged in the new scripts: their pages will be directly added to the section and not referenced as subscripts.
When inserting a subscript reference, the start-progress and end-progress attributes will be set based on the number of pages in the subscript and the total number of pages in the script, each page having the same weight.