As part of the "Pick a sentence" step (T251551) of the Section Translation mobile editor, a sentence will be focused for users to act on it.
Highlighting takes place over the section contents (loading and showing them is covered in T259499), and users can select a different sentence by taping on it. Other tickets will support the card with actions to act on these.
More details in the parent task: T251551: Section Translation Editor: Pick a sentence