a medical treatment apparatus , and in particular an extra - corporeal blood treatment apparatus , has a socket unit for the connection of a plug unit belonging to a device for supplying a medical fluid for the medical treatment apparatus . a filling apparatus for filling a device for supplying a medical fluid has a socket unit for the connection of a plug unit of the device for supplying a medical fluid . the device has a plug unit having means for indicating two states of operation , while the socket unit of the treatment apparatus has means for destroying the means for indicating two states of operation . when the plug unit is connected to the socket unit , the means for indicating two states of operation are destroyed , to rule out the possibility of the device being re - used . re - use of the device is ruled out by the fact that the device cannot be re - filled with fluid .