This instruction is part of the trio (with IPC and IPR) designed to provide safe calls between processes, almost as fast as function calls and without requiring microde.
The IPE instruction marks the entry point of a process and performs some security checks. If this instruction is not preceded by a NOP, or if this NOP is not jumped to from an IPC instruction, the process goes through a trap/exception. The trap is also triggered if the IPC jumps to another instruction than NOP, or this NOP is followed by an opcode different from IPE.
IPE is not interruptible or predicable (still for security reasons) and it writes the ID of the calling process to the destination register, so the calls can be filtered in software.