z segment definitions

Can anybody please help

I understand that if a z segment is at the end of a message that biztalk does not validate it if validation is switched on. However I have z segs inside the message rather that at the end as:
MSH EVN PID [{NTE}] [PD1] [ZPI] [ZEI] [{NK1}] [{OBX}]
etc

the parser errors finding OBX segs in z seg area. I understand then that I have to defile the z segs explicitly. I have had 2 goes at this now and the schemas have broken each time. Does anybody know how to do this. and are there any examples that i can follow.

Many thanks