The diagram illustrates how collaboration between specialist roles can develop step by step – from informal coordination to a fully-fledged cooperation group.
If two specialist roles from different groups are working on a shared topic, informal collaboration is usually sufficient. It is straightforward, flexible and requires little organisational effort. However, if the collaboration becomes more intensive, involves several specialist roles, or gives rise to shared technical, strategic or financial responsibility, it is advisable to establish a dedicated cooperation group. This creates clear areas of responsibility, binding decisions and a common working basis. The rule of thumb is: as much organisation as necessary, as little as possible. As long as informal coordination is sufficient, no new group should be formed. Only when complexity and responsibility increase on a permanent basis does a formalised cooperation group make sense.
Source: See here