A POPE is defined in Article 10 of the OECD Model Rules as a constituent entity, other than a UPE, that owns (directly or indirectly) an ownership interest in another constituent entity in the same group, and the right to more than 20% of its profits are held by persons that aren’t part of the MNE group.
There’s a departure here from the standard top-down rule for POPEs as under Article 2.1.4 of the OECD Model Rules, even if a UPE applies an IIR, if there is a POPE that owns an interest in a low-taxed entity it will also need to apply the IIR.