What do sub notes provided in a musical context refer to?

Sub notes in a musical context refer to notes that are shorter than standard durations. This term is used to describe notes that fall within the rhythm of a piece but have a reduced time value compared to the regular notes. For example, if a quarter note is the standard duration in a measure, sub notes might represent eighth notes or sixteenth notes, which take up less time within that measure. This concept is important for creating intricate rhythms and adding complexity to compositions, allowing for more dynamic musical expressions.

The other options do not align with the definition of sub notes. Footnotes in a textbook relate to additional information or citations, extended measures pertain to lengthening the number of beats in a measure, and chords played simultaneously involve harmony rather than the duration of individual notes.