When a map number appears only on one side of the reaction scheme, it marks an orphan atom.
An orphan atom on the reactant side is removed, an orphan atom on the product side is added during the reaction.
Take the reaction
Actual reaction according to the definition above
If a reaction includes substitution of a hydrogen on a carbon atom, the hydrogens should be expressed as explicit hydrogen atoms and marked as orphan atoms. Thus substitution only occurs on these explicit hydrogens.