Contents cover can be included for Landlords who rent out furnished properties giving peace of mind that belongings can be replaced in the event of a loss.
Insuring contents would not include the contents of your tenants, they would be responsible for insuring their own belongings. The contents you would be responsible in an unoccupied rental property would be for any removable kitchen white goods & appliances. Carpets would also fall under the cover of contents as they could also be removed from the property and is sometimes confused with being covered as part of the building.
The contents cover within a Landlord Property Owners policy would only cover the contents you provide, It would not provide cover the the contents that your tenants bring to the property, they would have to take out a separate policy for their own contents if they were to want protection for their own belongings.