The main reason for writing new 31-bit addressing mode programs that reside below 16 megabytes is to be able to address areas above 16 megabytes or to invoke 31-bit addressing mode programs while, at the same time, simplifying communication with existing 24-bit addressing mode programs or system services, particularly data management. For example, VSAM macros accept callers in 24-bit or 31-bit addressing mode.
Even though your program resides below 16 megabytes, you must be concerned about dealing with programs that require entry in 24-bit addressing mode or that require parameters to be below 16 megabytes. Figure 1 in Establishing linkage contains more information about parameter requirements.