Networks and addresses
PI and PA address space
Provider-independent space is registered to you and moves with you; provider-aggregatable space belongs to the provider's block and stays behind when you leave.
PA (provider-aggregatable) space is part of an ISP's or hosting provider's allocation. The provider assigns a piece to you, registers it in your name inside its block, and announces it as part of its larger prefix, or you announce it as a more specific with your own AS number. When the contract ends, the addresses return to the provider.
PI (provider-independent) space is assigned by the RIR directly to your organisation, through a sponsoring LIR. It is yours: it does not change when you change provider or sponsor, and you announce it yourself. In return the RIR's rules apply to it, including that PI may not be sub-assigned to other organisations.
The choice follows the use. A hosting company or ISP that hands blocks to customers needs PA, because PI forbids sub-assignment. A network that wants addressing that survives any change of provider wants PI. A /48 of either is enough for most sites.