Language Environment supports two forms of preinitialized environments. The first, supports the execution of main routines. The second, supports the execution of subroutines.

The primary difference between these environments is the amount of Language Environment initialization (and termination) that occurs on each application invocation call. With an environment that supports main routines, most of the application's execution environment is reinitialized with each invocation. With an environment that supports subroutines, very little of the execution environment is reinitialized with each invocation. This difference has its advantages and disadvantages.

For the main environment, the advantages are that a new, pristine environment is created. The disadvantage is poorer performance.

For the subenvironment, the advantage is that it provides the best performance. The disadvantages are that the environment is left in the state that the previous application left it in, and the runtime options cannot be changed.