2-Linked is a directory for your linked prose. It is within Org's secondary search scope, and is searched recursively.
Org link search finds GUIDs (Globally Unique IDentifiers) in
2-Linked. Org agenda search only finds headings in
2-Linked searches can rely on cached results, whereas
1-Agenda searches cannot. Therefore the former can store much more text than the latter, without causing excessive lag.
To read more about GUID link searches, go to by-Part/org-id