Assigning a unit means that the Tenant has permanently moved out of the unit and transferred the tenancy to another person. Terms of the original rental agreement remain the same (the amount of the rent and what services are included, etc.) and the new tenant is obligated to the landlord.
Subletting a unit, on the other hand, means that the Tenant has moved out of the unit for a specific period of time, but the Tenant plans to return before the end of his or her lease. The person who moves in is a subtenant or subletter. The subtenant pays rent to the original Tenant who then pays the Landlord. A Landlord cannot refuse the idea of subletting a unit, but the Landlord can refuse to allow the Tenant to sublet to a specific person, with good reason; however, if the Tenant thinks that the Landlord is being unreasonable in withholding consent to sublet to someone, the Tenant can serve a notice of termination and file an application with the Board.
If a Tenant assigns or sublets his or her unit without the Landlord’s consent, the subtenant is considered an unauthorized occupant. A Landlord can file an application with the Board to evict both the Tenant and the unauthorized occupant; however, if the Landlord does not file the application within 60 days of discovering the unauthorized occupant, the unauthorized occupant will become a Tenant.