How should relative-length values be interpreted when used in @font-face descriptors?

The grammars for @font-face descriptors, for example, 'size-adjust', allow the use of relative-length values due to the fact that calc() allows for them.

For example, using 'size-adjust' again:

@font-face {
   size-adjust: calc(50% * sign(10em - 2px));
}

So the question is, what does 10em evaluate to? @font-palette-values contains this text:

Math functions, such as calc(), and also var(), and env(), are valid within descriptor values in a @font-palette-values rule. They are evaluated within the context of the root element. Relative units are also evaluated within the context of the root element.

but I can't find any similar text for @font-face.