What is the difference between assigning a lease and subletting part of my space in Ontario?

Assignment and subletting both let a tenant bring in someone else to occupy leased space, but they work quite differently. In an assignment, the original tenant transfers their entire remaining interest in the lease to a new tenant, who takes over the tenant's position under the lease for the rest of the term, effectively stepping into the original tenant's shoes with the landlord going forward, though many leases keep the original tenant liable as a backup unless specifically released.

Subletting is different: the original tenant remains the tenant under the head lease and enters into a separate, new lease with a subtenant, for all or part of the space, for all or part of the remaining term. The original tenant becomes, in effect, a landlord to the subtenant while remaining fully responsible to their own landlord under the original lease, sitting in the middle of the two relationships rather than stepping out of the picture.

Both generally require the landlord's consent under most commercial leases, and the degree of ongoing liability the original tenant retains differs between the two. If you're only looking to give up part of your space, or want to keep some involvement, subletting may fit better than a full assignment; if you're leaving entirely, assignment is usually the cleaner route.