Back to wall pans are used together with built in vanity furniture. The WC or unit will contain a concealed cistern and pipework. Access to the cistern must be maintained, so most concealed WC units have a removable front panel and an access hatch in the counter top above the concealed cistern. Back to wall toilets are very neat as there is no visible pipework or cistern.
Wall hung, back to wall toilets require special bracing frames to be used when fitting the WC unit. Wall hung toilets give you more visible floor space. This can make a bathroom feel more spacious especially in smaller bathrooms.