function 

The function element is used to represent classical functional harmony with an indication like I, II, III rather than C, D, E. It is relative to the key that is specified in the MusicXML encoding.

Element Information

Model

Attributes

QName Type Fixed Default Use Inheritable Annotation
color color optional
default-x tenths optional
default-y tenths optional
font-family comma-separated-text optional
font-size font-size optional
font-style font-style optional
font-weight font-weight optional
relative-x tenths optional
relative-y tenths optional

Source 

<xs:element name="function" type="style-text">
  <xs:annotation>
    <xs:documentation>The function element is used to represent classical functional harmony with an indication like I, II, III rather than C, D, E. It is relative to the key that is specified in the MusicXML encoding.</xs:documentation>
  </xs:annotation>
</xs:element>

Sample