One of those 'no clear answer' world-view queries.
In gameplay terms, its defined by the Threat stat-block:
Beltinians can detect: Type: Undead, Subtype: Shade (see CSE:Threats chapter)
Between Worlds (ta) can detect: Type: Spirit (see Bestiary Vol.1)
According to some schools of thought, Elorii could be considered 'spirits' as they are elemental essence infusing a constructed body...
Note that Incorporeal is a Trait held by all of these creatures, but not unique to them (high level spellcasters and some weird creatures may also have it).