These are properties which are normally let to students or several professionals ,ie not a family, under one tenancy agreement. A very lucrative asset if you can get one but with these there are also stipulations. The rental income may be higher but the rental income required to obtain the loan amount you require is also higher but generally these are not too much of an issue.
Always best to speak with an adviser with regards to this as lenders generally require an HMO licence to be on the property on completion of the mortgage. Classification of HMOs vary from council to council in the UK and their criteria and stipulations do vary so please check with your local council if you need to know more. The council should be your first port of call but if not your adviser should be able to give you some advice on these type of properties.