The designator prevSibling describes the sibling note in $OutlineOrder before the note in current focus; the previous note on the same outline level with the same parent, i.e. excluding children.
Note that whilst prevSibling is the normal assumed spelling, the alternate of previousSibling is supported, though deprecated.
prevSibling is a more closely designed test than previous as the latter may be at a different outline depth.
prevSibling will not match an adornment, separator, or item set to not export but see previousSiblingItem.
See also—notes linking to here: