From Xojo Documentation
Name of the font used to display the caption or text content.
You can enter any font that is installed on the computer or the names of the two metafonts, "System" and "SmallSystem".
The System font is the font used by the system software as its default font. Different operating systems use different default fonts. If the system software supports both a large and small System font, you can also specify the "SmallSystem" font as your TextFont.
On Macintosh, "SmallSystem" specifies the OS's smaller system font and may make the control smaller in size as well. On Windows and Linux, "SmallSystem" is the same as "System".
Classes implementing the TextFont property
This code sets the TextFont property.