Sub-topic label

Sometimes I use relations to connect different elements and it allows to insert a label which is very useful. In some cases I don't want to create a relation, but I still would like to name the line that connects a topic to a sub-topic for example, because a  sub-topic could be just a part of the parent topic or it could be related to the parents via another relation that I want to specify. It would be useful to write a label like a verb or something else to connect topic and sub-topic above the connection line itself without creating an arrow-like shape for a relation.