Hi,
I think this person was actually asking about arranged employment opinion (AEO) (see the title of this topic, but I could be wrong), which is issued by HRDC once they assess the prospective future employer and the genuiness of the job offer (offered wages and so on...). Without a valid AEO the applicant will not be able to claim 10 points in the arranged employment category as a skilled worker for immigration purposes. In this category there are no exemptions. AEO is valid up to 5 years.
http://www.hrsdc.gc.ca/eng/workplaceskills/foreign_workers/aehrdcassess.shtml
http://www.cic.gc.ca/english/immigrate/skilled/apply-who-employment.asp
LMO is for the work permit and it is not for immigration as a skilled worker, it is only for a temporary worker. Here in this category there are exemptions as you posted.
LMO is valid for only short time ( not sure maybe 6 month, need to check )
http://www.hrsdc.gc.ca/eng/workplaceskills/foreign_workers/index.shtml
Just to avoid future confusions.
HTH