Related questions and search terms for this same question are: What is fontzen please?
Fontzen is a reference to one of the available font-slots that you can configure. For example a text widget has a title fontzen property that allows you to determine what font should be used for the title when its being rendered on the page. If the first font-slot is configured to use the "Arial" font, and if a text widget's "title fontzen" points to that first font-slot, then that title will be rendered using the "Arial" font. Its a best practice to limit the number of fonts you use on the website, and therefore we have limited the amount of available font-slots to 3. This is explained in more detail in this support video;
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