Warning_@startsection

Class scr… Warning: `\@startsection' has been changed. …

The whole message in case of using scrartcl would be something like:

Class scrartcl Warning: `\@startsection' has been changed.
(scrartcl)              scrartcl needs it's own definition of `\@startsection'
(scrartcl)              to provide extended features for the optional argument
(scrartcl)              of `\section' etc.
(scrartcl)              Generally it defines `\@startsection' completely new
(scrartcl)              to achieve this. Because of the unexpected definition
(scrartcl)              an alternative approach will be used.
(scrartcl)              If this fails and if there isn't a new release of
(scrartcl)              KOMA-Script that fixes the problem, please
(scrartcl)              send a report to the KOMA-Script author.
(scrartcl)              Note, that this alternative approach does not
(scrartcl)              execute the elements of do-hooks:
(scrartcl)                - `heading/postinit',
(scrartcl)                - `heading/branch/star',
(scrartcl)                - `heading/branch/nostar',
(scrartcl)              and does always use `runin=bysign' and
(scrartcl)              `afterindent=bysign'.

As the warning explains, the KOMA-Script classes do not use the original \@startsection internally to define headings, but rather a modified version. However, this modified version does not work if tagging is enabled with \DocumentMetadata or by other means. Other changes to LaTeX core or through packages can also change the definition of \@startsection. To minimize the risk of malfunction, the KOMA-Script classes then revert to a different mode of operation. However, this means that some extensions of the KOMA-Script classes can no longer be used.

For example In the case of tagging, there are further warnings:

Class scrartcl Warning: Unexpected definition of \@sect!
(scrartcl)              Please send information about this to
(scrartcl)              the KOMA-Script maintainer!
(scrartcl)              Maybe LaTeX will be broken by the redefinition
(scrartcl)              of \@sect on input line 3549.


Class scrartcl Warning: Unexpected definition of \@ssect!
(scrartcl)              Please send information about this to
(scrartcl)              the KOMA-Script maintainer!
(scrartcl)              Maybe LaTeX will be broken by the redefinition
(scrartcl)              of \@ssect on input line 3745.


Class scrartcl Warning: Unexpected definition of \@xsect!
(scrartcl)              Please send information about this to
(scrartcl)              the KOMA-Script maintainer!
(scrartcl)              Maybe LaTeX will be broken by the redefinition
(scrartcl)              of \@xsect on input line 3826.

These are more serious, because KOMA-Script cannot revert to a different mode here, but need to redefine the three commands and this redefinition indeed can break something.

In following cases you don't need to send any information:

• Using \DocumentMetadata with activation of tagging.
  The KOMA-Script classes currently do not support tagging. The work involved is anything but trivial and cannot be done by the main KOMA-Script developer alone. I also do not consider it useful to support tagging in only a few places, for example in headings from \section to \subparagraph. Official support for tagging simply takes time, which I cannot spare at the moment. Those who rely on tagging are well advised to use the standard classes and only a few packages.
  See also: [#89] [#59].

Note: The warnings only exist in KOMA-Script up to and including version 3.48.