To get back to topic.
The Link Pack Area (LPA) is a collection of programs and fixed tables that appear to be programs that are loaded into storage when the system is started, or to use our quaint term, IPLed. These programs and tables can be used by any program at any time.
There are several minor variations.
The "fixed" LPA (FLPA) has its pages arranged so the system will always have them in main storage. Pages assigned to the "regular" LPA can be paged out of main storage.
The "modified" LPA (MLPA) has its pages arranged so they can be altered. Pages in the regular LPA cannot be altered. This capability has always seemed stupid to me, but there are a few modules where this is a requirement.
Given that many z/OS systems are rarely IPLed these days often means that using LPA is not always a good idea, since the only way for an update to "take" is to IPL the system. For this reason user programs are rarely placed in the LPA; it's just for IBM provided programs.