Update - version 1.2.0:- Added support for devices with multiple profiles (now all the data will be synced with the current profile instead of the default one).
- Added ability to sync reading position (if you have the Coolreader app installed on Pocketbook, you can sync Coolreader's position too).
About syncing of reading positions:
Positions are stored as plain text in yet another custom column (can be text or comments type). There is no way to use these positions in Calibre or for syncing with different apps. I made this functional mainly for backup, because sometimes reader erases saved positions, for example when you move your books around, etc. Now they can be preserved in Calibre and easily restored when lost.
You can save positions for the main Pocketbook reader app and for Coolreader app. They are using different formats, so you will need two separate custom columns, if you want to save both. If you only need to save positions for one app, remove the column name you don't need in the plugin's settings.
Syncing never ever deletes any saved position. If you have position in the reader and don't have in in Calibre, it would be added to Calibre, and if you have in in Calibre and don't have it in the reader, it would be added to the reader. If both Calibre and the reader have saved positions, the one with the latest timestamp would be added to both. This way you will always have your latest positions saved.
Keep in mind then in Pocketbook reading position and completion mark are connected, so you will lose you completion mark if you open the book in PB app with position less then 100%. Coolreader's position do not affect completion marks.
This new functional is rather experimental, especially with Coolreader, because it stores it's positions in xml instead of database. So let me know if there is any bugs.