Final answer:
Liam and Emma are illustrating the concept of Boundaries by deciding not to discuss past relationships, showing the importance of setting limits and respecting them for healthy communication in relationships.
Step-by-step explanation:
Liam and Emma have decided that past relationships are a taboo topic and one that is not to be discussed in the relationship. They are illustrating the concept of Boundaries. This decision indicates that they have set a limit on what they believe should be open for discussion between them, which is a critical element for the health and functioning of any relationship. Establishing boundaries can help partners feel safe and respected, and it is an important aspect of communication in any relationship. While every relationship may involve different boundaries, the key is that both partners agree on these limits and respect them, which further contributes to a healthy partnership.