POWER II, the spooling system IBM provided as a separate package for use with DOS/360, runs in F1 or F2 (usually F1). POWER needs to run in a higher-priority partition than the partitions it serves, so if POWER ran in F1, it could provide spooling services to BG and F2; a POWER system generation parameter determined if F2 received spooling services from POWER. The result, whether F2 received spooling services, is that POWER II turned a three-partition DOS/360 system into a two-partition system.
POWER was a pretty primitive spooling system--albeit much better than what passed for spooling in the 70xx/14xx computing system days. But it was not better enough to keep people from thinking that they could write a better spooler, and charge others for using it.
GRASP and The Spooler were competing spooling products that cost money to license, and it says something that a rational Data Center manager spent money on a capability--spooling--that could be had from IBM for free. But these packages did well in the nascent marketplace for pay-for licensed software.
Based on my personal experience, GRASP did many things better than POWER II. For example, GRASP created a fourth partition, F0, and a 50% improvement in available processing partitions (from two to three) was apparently worth some cash. I ran GRASPin F0, Minicomm, a teleprocessing monitor, in F1, batch in BG, and utilities and RJE terminal software in F2. This was 35 years ago, so some details may be wrong, but that's the idea.
Trivium: POWER is a forced acronym (Prioritized Output Writers, Execution processors, and Readers), perhaps similar to SPOOL (Simultaneous Peripheral Operation On-Line).
The generally available POWER distribution is really for DOS release 27, the last release of DOS that did not include virtual storage support. This distribution includes 3330 DASD support, which DOS release 26.2 does not.
(POWER/VS, a rearchitected and vastly improved product, was delivered as an independent component for DOS/VS release 30 and was integrated into DOS/VS release 31. After DOS/VS release 34, it became the POWER/VSE licensed program product. POWER/VS and its successors were the generally accepted spooling packages for DOS/VS and its successors. And DOS/VS releases supported additional partitions, so losing one to a spooling system was much less of a problem.)
POWER II is an interesting proposition under Hercules, which provides its own excellent set of spooling capabilities. POWER II might seem a bit redundant. Hercules delivers spooling through specification of additional virtual unit record hardware. In contrast, the 1960's era Data Center manager could not rationally purchase an additional set or two of unit record hardware to support batch processing in F1 and/or F2.
That said, a DOS/360 system using POWER II is a bit easier to use under Hercules than one that does not because POWER II supports a hot reader and DOS/360 by itself does not. There is less console interaction. But decide for yourself.
If you've never used a mainframe before (real, virtual, or emulated), continue here. If your mainframe experience is limited to OS/360 or its successors, read a comparison of DOS/360 and OS/360. Otherwise, your next step is to review the DOS/360 distribution.