Assigning a unit means that the tenant moves out of the unit permanently and transfers their tenancy to another person. All the terms of the original rental agreement stay the same – for example, the amount of the rent and what services are included.
Subletting a unit means that the tenant moves out of the unit for a period of time but plans to move back in before the end of the tenancy. The person who moves in is known as a subtenant. That person pays the rent to the original tenant who then pays it to the landlord.
A tenant must have the landlord’s approval to assign or sublet the unit, but the landlord must have a good reason to refuse.
For more information see the brochure: How a Tenant Can End Their Tenancy.
Source: www.sjto.gov.on.ca