How Label's FontSize property can accept integer and NamedSize values in the same time in Xaml?

Hi,

I try to create my own custom control. And I need a FontSize property for my control like the Label has. But in the Label this property has a type of 'double'. And I don't understand how can we write both integer and NamedSize values in Xaml in the same time.

<Label Text="Biggest NamedSize" FontSize="Large" />
<Label Text="Use size 72" FontSize="72" />

How it works? How the Label understands both values of size? Can anyone explain me?
Thanks in advance!

Best Regards,
Aleksandrs