There are different sets of function suites available to Premiere plugins. SweetPea Suites are the more modern suites that have been added for most new functionality. The piSuites are still needed for various functionality that has not all been superceded by SweetPea Suites.
Whenever possible, use SweetPea Suites.
There are also function suites more specific to certain plugin types. The Bottleneck Functions are useful for transitions and video filters. Other suites available to only one plugin type are documented in the appropriate chapter for that plugin type.