Within the browser view, the single input to a Species element is labeled "Definition". It is a vector by species. It expands to show each of the species in the model. Of all of the properties of a particular species, only two accept links from other elements: the Half-life (or Decay Rate) and the Stoichiometries for the various daughters. Therefore, if you expand a particular species of the Definition input, only these inputs appear:
The Half-lives (or Decay Rates) and Stoichiometry are the only items in browser view for a Species element that you can link an output to (using the Link Cursor). All other Species element inputs must be specified via the species definition dialog.