USFM3 includes some new markers for Ruby glosses (a special kind of annotation in Japanese), a new way of storing figures, and a few other new things. Details here
Versions of Paratext 8 before 18.104.22.168 (released May 2019) cannot read USFM3 projects. Recent updates to Paratext 8 can open these projects but the new markers might not display correctly. For instance, a figure inserted into a USFM3 project by Paratext 9 won’t be visible or editable in Paratext 8. You will see the info that a figure has been inserted but you will not see the figure in Preview or any other view.
When you create a project in Paratext 9, the default is to use USFM3 but it gives you this warning dialog before creating the project:
The warning lets you change the project properties to USFM2 if you need to be sharing the project with Paratext 8 users. You can also specify USFM 2 when creating the project by going to the Advanced tab of Project Properties.