Unless your tenancy agreement forbids subletting, your tenant can sublet rooms in the property. If your tenant sublets, the property could become a House in Multiple Occupation or, if it is already a HMO, subletting could lead to your HMO becoming overcrowded.
Not everyone who pays rent for a room or a property is a tenant. Some only have a license to remain in that property and are “licensees”. Licensees don't have as many rights as tenants and it's much easier to evict them.
Housing Executive tenants have rights which are set out in law. You are an introductory tenant for the first year of your tenancy. After 12 months you’ll become a secure tenant. Secure tenants have stronger rights than introductory tenants.