Because of changes in the way Experience Manager and Assembler process rules, the Presentation API is no longer suitable for writing full applications. Instead, use the Assembler API.
PAPI can be used to implement specific specialized features in certain circumstances, such as:
Precisely targeted Dimension Search queries, to serve as an alternative to how the reference applications use Assembler for typeahead. This can provide a performance advantage because Dimension Search queries do not require rules processing.
Implementing queries that retrieve specific product information, such as which category a product is in.
For information about PAPI, refer to MDEX Presentation API for Java Reference (Javadoc).