In the release notes for webMethods 10.2, it reads:

But there are not much information otherwise, screenshots, videos on youtube etc. What is meant by “coverage reports for suite coverage”? Is it code coverage at flow step level? Has anyone used the new feature and can provide more information about how easy it is to use, how the reports look like etc?

You can check the following documentation for some reference:
http://techcommunity.softwareag.com/ecosystem/documentation/webmethods/wmsuites/wmsuite10-2/Cross_Product/10-2_Unit_Test_Framework.pdf

Here, check Code Coverage Analysis section.

Refer this - Test Suite Executor
There is a topic on code coverage feature as well.

Thank you for the hint. We have already implemented the feature for our needs (without wM test suite).

wow interesting!!