Halliday & Matthiessen (2014: 624-5):
Exophoric reference means that the identity presumed by the reference item is recoverable from the environment of the text… . Here the reference links the text to its environment; but it does not contribute to the cohesion of the text, except indirectly when references to one and the same referent are repeated, forming a chain. Such chains are common in dialogue with repetition of references to the interactants by means of forms of I, you, we…
To be clear, the cohesive reference relation obtains between a reference item and its (endophoric) referent, not between reference items or between referents. A chain of reference items does not represent the cohesive relations of a text.