A scratch vocal is a vocal performance recorded by a singer in order to provide reference to a producer when crafting other pieces of the recorded song. It is used primarily for timing purposes, but may also be used to get an idea of how the vocalist will ultimately perform the vocal, in order to add embellishments in the instrumentation

Instrumentation (music)

In music, instrumentation refers to the particular combination of musical instruments employed in a composition, and to the properties of those instruments individually...

 that complement the singer's vocal inflection.

A scratch vocal is often recorded quickly and the singer may mark the song rather than hit more difficult notes. The singer will ultimately come back and re-record the vocal performance .