The word ‘Pastille‘ has evolved to mean many things over time. In candy and confections, it has two principal meanings: a small, hard, round or oval fruit candy, often used as a lozenge, and a disk of chocolate.
The pastille was popularized by Droste with a unique shape intended to fit into the soft palate: a rounded top of the pastille intended for the roof of the mouth, and a smooth flat bottom on which the tongue could gently press the chocolate upwards. With this configuration, the pastille fits, and melts, perfectly in the mouth.
Quality manufacturers such as Hachez make flat disks. Haigh makes traditional shapes.
Pastilles are often used by cooks for melting, to be used in other recipes.