This is used to identify the columns that will be returned, so some columns must be specified in order for data to be returned. If you state Nullable = false, then the column must exist in the list in order to be retrieved (just like our schema). This does not support Multi Lookup and Person/Group Fields. All columns referenced here must use the column’s internal name.
Example:
<FieldRef Name="FileRef" Nullable="TRUE"
/>
<FieldRef Name="LinkFilename" Nullable="TRUE" />
<FieldRef
Name="FileLeafRef" Nullable="TRUE" />