A note-to-note at note level, i.e. not from within $Text. This links the entire note with another note, or a text selection in another note. Basic links occur only within the current document, i.e. they can only begin or end at a note in the current TBX.
Originals and aliases export their own basic links (i.e. aliases can differ), but from v5.9.1 if an alias has no in/outbound basic links it will export those of the original.
Tinderbox provides many ways to generate basic links: