Explain why somatisation is described as a pattern of presentation within depressive illness specifically, rather than a separate diagnosis in its own right.

Somatisation, as described here, is the pattern by which an underlying depressive illness is experienced and expressed largely or entirely through physical bodily symptoms rather than through the emotional language of sadness or low mood, so the underlying condition remains the depressive illness itself, with somatisation describing how it presents rather than what it is. This differs from Briquet's syndrome, a separate, chronic, multi-system condition beginning before age 30 that unfolds independently of any single depressive episode, which is why the two should not be conflated despite superficial similarity. Recognising somatisation as a presentation pattern within depressive illness, rather than a standalone diagnosis, matters clinically because it directs the clinician back toward identifying and treating the underlying mood disturbance, rather than pursuing extensive investigation of the physical symptoms alone.