The ancillary_variables attribute of each field.
fl.ancillary_variables is equivalent to cf.List(f.ancillary_variables for f in fl).
fl.ancillary_variables = value is equivalent to for f in fl: f.ancillary_variables = value.
del fl.ancillary_variables is equivalent to for f in fl: del f.ancillary_variables.
See cf.Field.ancillary_variables for details.