They're apt to have their variations (6)
I believe the answer is:
themes
'variations' is the definition.
The answer and definition can be both related to communication as well as being plural nouns.
Maybe they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for themes that I've seen before include "'Subjects, topics (6)'" , "Subjects or repeated melodies" , "Subjects or repeated musical pieces" , "Motifs, subjects" , "Subjects or unifying ideas" .)