The TCPIP.DATA data set is also known as one of the resolver configuration data sets. In fact, this name is now more commonly used to refer to this important file in the UNIX System Services environment because the socket library contains a component called the resolver. In a UNIX system, you use the /etc/resolv.conf file for the same purpose as you use TCPIP.DATA in your MVS™ system.
TCPIP.DATA specifies the name of the TCP/IP address space. Because the data set search order can vary, your installation will determine which data set you can use. See IP configuration overview for search order, data set, and file retrieval information.
If you use TCPIP.DATA, it can be shared between multiple systems with a system name. But, if TCPIP.DATA is allocated by SYSTCPD DD and an application forks, any allocations from the parent of SYSTCPD are lost to the child process.
In z/OS® UNIX System Services, each application that uses the XL C/C++ API can have its own environment variable, RESOLVER_CONFIG='xxx'. There are no concerns for forked child processes; however, this means that you cannot share the data set or file among multiple systems.