Sorry for using the ALS nomenclature. I assumed "everyone" would know what was meant by Architecture Levelset (AKA ALS).
It means, for z/OS, that a release requires a specific set of hardware features. Generally, that is enforced, so IPL will fail if not met. Those features are usually provided by a particular generation of IBM machines. But the ALS is to the list of features, not to a machine. This usually relates to such complicating factors as running z/OS under z/VM or on zPDT which might not support everything that a machine provides. z/OS 1.6 was an ALS to base z/Architecture. z/OS 2.1 was an ALS to features provided in the z9 machine z/OS 2.2 was an ALS to features provided in the z10 machine z/OS 2.3 was an ALS to features provided in the zEC12 machine. Information about the features associated with each ALS is intended to be put into one of the books in the next release (planning for migration?). It was intended to have been surfaced in the announce material for each release that had an ALS. I have no idea if that happened or not. Peter Relson z/OS Core Technology Design
