PI and PI_BASIS are two different components.
PI is the abbrevated form of Plug In. AddOn components or extension sets are installed above core SAP and they integrate or communicate with the core using PlugIn component.
PI_BASIS is the core component or a single version for different PlugIns.
Hope you can understand what I meant in the answers above.
PI is the plugin to "plug" BI/BW and CRM to the system, it contains the extractors for BI and the qRFC part to sync CRM and R/3 (including the filters).
PI_BASIS is the plugin containing BASIS functionality (ALE p. ex.) It is planned to be able to also modularize this so that you can update PI_BASIS without doing a full upgrade but only upgrade this separate package through transaction SAINT.