PPSE
The directory a contactless terminal reads first to find out which payment applications a card offers.
It stands for Proximity Payment System Environment. The terminal sends a SELECT command for the name 2PAY.SYS.DDF01, and the card replies with a list of its application identifiers, each with a priority. From that list, and the AIDs it supports itself, the terminal picks an application and hands off to the matching EMV kernel.
Contact cards have an equivalent called PSE, named 1PAY.SYS.DDF01, but there the directory is optional and the terminal can simply try AIDs one by one. Over the air there is no time for that, so for contactless the PPSE is mandatory in practice. It is also the first place to look when a mobile wallet suddenly refuses one particular network.