Basic explanation of an EIB/KNX installation
The EIB/KNX bus is a system for building automation. All devices on this bus listen to all data packets and take only the data they are interested in. To do so, all data is addressed to "groups". Group addresses are normally written as a triplet of numbers, e.g. "1/2/3". Each object on the bus gets at least on unique address. E.g. a lamp could react to group address "Lamp office" and "All lamps on 2nd floor".
In every device, each group may have several attributes. The most important ones are "writable" and "readable". The latter one has to be set, if iKNX should be able to read the status of the object. In iKNX's settings for each group, you can deactivate the reading of an object's status - important if a group address is used by a lot of devices (e.g. group "All lamps on floor 2").
An more comprehensive description of the bus can be found on Wikipedia.