Effect of FontSize as a class parameter

[ehud.behar at protonmail.com]

What is the meaning of the class parameter FontSize when inserting it in a layout file inside ClassOptions?

The customization manual says:
FontSize [string="10|11|12"] The list of available font sizes for the document’s main font, separated by “|”.

Does this command takes effect only inside the LyX editor? If so, where and how?
Or rather does it make any changes to the latex preamble? I haven't seen any.
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