Most Veritas Cluster Server implementations attempt to build availability into a cluster, eliminating single points of failure by making use of redundant components like multiple network cards, storage area networks in addition to the use of VCS.
The basic architecture of VCS includes low-latency transport (LLT), the Global Membership Services and Atomic Broadcast Protocol (GAB), the High Availability Daemon (HAD), and Cluster Agents.
LLT lies at the bottom of the architecture and acts as conduit between GAB and underlying network.
Each of those cluster services need to register with GAB and is assigned a predetermined unique port name ( a single alphabet).
In order for applications to be able to plug into High Availability Framework, it needs Cluster agent software.
For example, for Oracle to utilize HA (High Availability) framework in VCS, it needs an agent software.
VCS at base is generic Cluster software and may not know how different applications start, stop, monitor, clean etc.