Futon and Sofa bed

Futon noun - Mattress consisting of a pad of cotton batting that is used for sleeping on the floor or on a raised frame.

Sofa bed and futon are semantically related In some cases you can use "Sofa bed" instead a noun "Futon".

Sofa bed noun - A sofa that can be converted into a bed.

Futon and sofa bed are semantically related in couch that becomes bed topic. You can use "Futon" instead a noun phrase "Sofa bed".