"Formula milk" refers to a specially made type of milk for babies who cannot have breast milk. It is a nutritionally balanced substitute made to provide the necessary nutrients required by a baby's body for growth and development. Full definition
The composition of breast milk differs from that of formula milk in several respects. (academic.oup.com)
Unless there are other reasons, you do not need to supplement baby with formula milk if they are feeding regularly and passing plenty of wet and dirty diapers. (expressing-mama.com)