What the authors above are actually referring to is probably
the percent of all babies in the age range 0 -
5.99 months who received no food or fluid but breast milk
the day before a questionnaire was administered to their
mothers. This is the way WHO originally advised that data
on exclusive breastfeeding be reported [6]. Thus a 24-hour
recall is done with mothers of all infants 0 - 5.99 months
of age to ask what they fed the infant the day before the
survey. The number who said “nothing but breast milk” is
divided by the total number, resulting in a proportion
(usually turned into a percentage).