Combined restoration is a combination of an un-removable and a removable restoration. In this case an un-removable and invisible device, the so called precision attachment, is placed in the mouth. The precision attachment itself has two parts: one is fixed to a crown in the mouth, while the other is the part which actually holds the removable partial denture.
The precision attachment cannot be removed by the patient, because it has to be firm to give the required comfort and safety. Of course if for some causes the un-removable attachment has to be removed, your dentist can take it out anytime.
The removable part of the combined restoration au contraire should be removed by the patient every day. The partial denture has to be cleaned and taken care of every day. The un-removable part of a combined restoration is designed to last long, withstand and resist, while the denture itself is made of materials that secure the feeling of comfort, and thus has to be of weaker materials.