Complex sentence: Message boundaries are triggered by the application using UniPro and signaled via an end-of-message bit in the segment header: the bit indicates that the last byte in the L4 segment is the last byte of the application-level message.
Split into simpler sentences: Message boundaries are triggered by the application-level protocol using UniPro and are signaled via a bit in the segment header. This End-of-Message bit indicates that the last byte in the L4 segment is the last byte of the application-level message.