The description pretty much covers it.The documentation for home-fontconfig-service-type¹ (as of 2025-01-15) does not specify the format for the extension list. Specifically, the sublist of the example list appears to expect certain elements which are not explained, nor does the page link to other documentation which explains them.
¹ https://guix.gnu.org/manual/devel/en/html_node/Fonts-Home-Services.html
