There are 5 persons editing the same Word document. I want to see who edited one part of it. Is that possible in SharePoint online?
Well Sharepoint will record Version History (if enabled in the Document Library Settings).
However, it seems like you need to use Track Changes in the Word settings. This allows multiple users to work on a file e.g. a report, Word records 'who did what'. You'll need to familiarise yourself with the settings to see how it behaves.
However, if you really need something in Sharepoint to track this, then Sharepoint will only record data from the 'properties' e.g. Created, Modified Date, plus Version History - the latter will record when fields were changed.
You can create custom fields to track certain information, you might be able to configure something whereby a document editor can only see the file assigned to them. Sharepoint will track changes made to the 'Assigned To' people field, therefore you would gain some visibility on the person responsible for created that version of the document.
You may also wish to investigate using an 'Approval Workflow' for document management.
Just found this one - very relevant.