I've come across these terminologies—threshold symptoms and sub-threshold symptoms—in one of the papers in psychology where delayed reactions are introduced:
Empirical studies that have mapped PTSD symptoms over time in fact observed what appear to be delayed elevations in the direction of threshold symptoms...
However, I don't have any ideas what those words are all about.
I believe that the concept of 'subthreashold symptoms' refers to a situation where a client clearly presents with symptoms of a given condition but, crucially, they present with too few symptoms to meet the diagnostic criteria.
This link, which hopefully also satisfies Seanny123's request for a document, states:
Although DSM-IV acknowledged the clinical significance of some subthreshold forms of unipolar depression, such as minor depression (MinD) and recurrent brief depression (RBD), clinicians continued to struggle with the concept of "subthreshold" depression. A substantial number of patients continued to present with depressive symptoms that still did not satisfy any DSM-IV diagnosis. Generally, these patients failed to complain of anhedonia and depressed mood, a criterion that DSM-IV mandates for any diagnosis of depression.
