In this case, words are omitted when the phrase would otherwise be repeated.
For example: The boy and the girl went to the shop where they bought an ice cream each. The boy bought a chocolate flavoured ice cream and the girl bought a caramel ^ one. He asked for his ^ in a cup, she ^ a cone. Afterwards, they went to the park.
^ The words flavoured, ice cream, and asked for hers in a have been omitted.