PA addresses are assigned from an LIR's allocation and are registered in the RIPE Database by the LIR. The advantage of PA addresses is that the routing information for many customers can be aggregated once it leaves the provider's routing domain.
PI address space is assigned separately and not from an LIR's PA allocation. All PI assignments are registered in the RIPE Database by the RIPE NCC at the time they are assigned. PI assignments are usually small; they cannot be aggregated into larger blocks. The disadvantage of this is that networks operators throughout the Internet may choose not to route them.
As the RIPE NCC has run out of IPv4 addresses, no new IPv4 PI address space can be assigned. IPv6 PI address space is still available.