If you need to have your KMail filter settings available on another system, you’ll have to export those settings from KMail and then, import them into the KMail executing on the other system.
Are they all the KMail cofiguration files needed? Or may I miss anything else?
And, instead of exporting and importing, will it work to move all those files to a Nextcloud server and then create symbolic links to them in my two computers which run the same system and KMail version?
The data storage access mechanism for all PIM (Personal Information Manager) data in KDE. This allows various applications to access the required information in one place. Note that use of Akonadi does not change data storage formats (vcard, iCalendar, mbox, maildir etc.) - it just provides a new way of accessing and updating the data.
The main reasons for design and development of Akonadi are of a technical nature, i.e., having a unique way to access PIM-data (contacts, calendars, emails…) from different applications (KMail and Calligra, for instance), thus eliminating the need to write similar code over and over again.
Another reason is to de-couple GUI applications like KMail from directly accessing external resources like mail-servers – this was a major reason for bug-reports/wishes with regard to performance/responsiveness in the past.