UNIX systems limit how much of various system resources each process on the system can use. This prevents badly-behaved processes from using all the system resources and tying up the machine so that no other processes can run. In some cases you might need to configure the operating environment for Enterprise Server so that it can use more than the default amount of resources.
Which resources are limited, the default limits for them, and the maximum limits supported by the operating system depend on the version of UNIX and how it is configured on each system. See your operating system documentation for more details. Typically, UNIX systems can limit the memory, the amount of CPU time, and the number of open files (including network sockets) used by each process, and the size of files it creates.
An Enterprise Server instance might run short on resources if it is required to handle a large client load. The Enterprise Server component which is most likely to encounter resource limits is the Micro Focus Communications Process, mfcs32. There is one mfcs32 process for each started communications process in the started enterprise server.