How does z/OS know to look for this member CAISECxx for starting ACF2?
Component : CA ACF2 for z/OS
z/OS knows nothing about CAISECxx. The ACF2 code that is in the LPA libraries knows about CAISECxx and looks for it. Using CAISECxx is the recommended way to start ACF2. Not using it may cause unexpected IPL problems, like SEC6 in OMVS for example.