Where, by which way, and by whom is the windowClass string defined ?

It is accessible by

wmiface windowClass windowId

which I use.

(And also by

Right click on window bar
Advanced
Special settings
Window
windowClass (application type)
)

It is the only way I have found to get the application name from the windowId.
windowClass often contains the application name, but not always.
The content format/convention is inconsistent (very variable).
To have a consistent mapping, I whould have to build a table.
To build that table I need to know where it is defined.

Thanks.

Whilst I do not know the specifics of this, I would guess, given that it differs from application to application, that it can be specified by the application itself. I would recommend asking the KWin developers on their mailing list (kwin@kde.org) however.