It is a variant of TPF specially designed to provide all the benefits of TPF (very high speed, high volume, and high availability in transaction processing) but with the advantages such as easier integration into the data center offered by running on a standard IBM operating system platform.
Because it runs under standard IBM operating systems, it can easily leverage other developments or enhancements on those platforms.
For example, to enable MQSeries for TPF required the porting, modification, and authoring of large volumes of code.
To enable this on ALCS, only an interface to the MQ product on z/OS needed to be written.
This also applies to TCP/IP, Db2, VTAM, to a certain extent language support and even basic disk I/O.