What is the "Diet" Field in Create?
Create by default outputs Recipe Schema markup in any post where you embed a card. Like all Schema categories, Recipe Schema requires or requests certain information to be included so search engines, like Google, can understand your content better and make your post eligible for Rich Results.
One of the requested properties for Recipe Schema is Diet, the official property name being "suitableforDiet." According to Schema.org, Diet "Indicates a dietary restriction or guideline for which this recipe or menu item is suitable, e.g. diabetic, halal etc."
Because Recipe Schema requests a Diet, the Create plugin provides a Diet field in Recipe cards.
The drop-down menu allows you to select one of 12 Diet options available. Those options are:
N/A (choose this option if your recipe does not adhere to any specific restrictive diet)
Why Are These the Only Diet Options Available in Create?
Along with requesting that a Diet be specified in your card, Recipe Schema allows only a limited number of Diets as options. This means that Google only recognizes those specific Diets for Schema purposes. In order to adhere to Schema requirements, Diets that aren't included are therefore not options in the Create plugin.
If Google decides to recognize additional Diets for Recipe Schema in the future, then they likely will be added to the Create plugin!
If your preferred Diet is not available as an option in Create, you can select "N/A." If another available Diet applies to your recipe, you can select that option instead. For example, though Low-Carb is not a recognized Diet option, if your recipe is both Low-Carb and Gluten-Free, you can select the Gluten-Free option if you wish.
Although Google does not recognize all Diets as far as Schema is concerned, Schema is simply one facet of SEO, and Google can and will promote great content for any kind of diet.