DITA2Go assumes a reltable describes all relationships you might ever want for topics in the map; or potentially in the map, perhaps under different conditions.
This “permissive” view of reltables allows them to provide a “superset” of relationships, from which DITA2Go produces links whenever they make sense: that is, when both source and target are part of the navigation hierarchy for the current project. This vastly simplifies re-use of reltables.
DITA2Go silently omits any reltable links where the link target is not included as content in the root map or in any maps referenced from the root map.