Single Bookmark:

It can contain only one bookmark. So, if a new bookmark is created for this type, it will replace the previous one (if any). That might be helpful to manually mark last read ayah.

Multiple Bookmarks:

It can contain multiple bookmarks. So, several bookmarks can be listed under a category. That might be helpful to list several ayahs under a certain category like rewards, punishment, salah etc.

We plan to rephrase the Single bookmark to Pins and Multiple bookmark to Collections for better user experience in the future, in sha Allah.