It’s well-known that any digital bus transfer signals are characterized only by two electric states - "0" and "1" ("Low level" and "High level"). The state when both lines (SDA and SCL) are set to "1" is considered the free state of the bus. The bus at this point is not busy and ready to transfer the data (in other words, the bus is standby mode). But how do we ensure this state if the interface elements do not have any active outputs? To provide the logic states, the bus is connected to two external resistors (pull-up resistors), pulling up the line to the power supply voltage U. Typical values of these resistors range from 1 to 10 kOhm. Sometimes these resistors are a part of a master subscriber. In general, it’s desirable to study the interface of a master subscriber thoroughly, since its outputs can be active (they can have transistor switches instead of pull-up resistors) and there is a conflict on the bus in such a case.
Here we can clearly say what the main difference between a master-subscriber and a slave-subscriber is. It's very simple. Only a master-subscriber can generate an SCL signal.
The most important criterion which determines the use of a particular bus is its range of specifications. The 12C bus is a bidirectional asynchronous bus with a serial data transmission and, therefore, has a fairly low bandwidth. That is why it is hardly ever used with personal computers. It can be used as an auxiliary for the identification of installed devices. But it is quite good for the operational coordination of the devices in home appliances.
The 12C bus is not suitable for the connection between remote subscribers, and thus can only be used as a part of one particular device. Mainly, this conclusion is based on the fact that SDA and SCL lines are short-haul lines. An article was found on the Internet where the author said that his design operated properly with a bus of a total length of about 100 meters (of course, with decrease in communication speed). But this mode is not normalized by the original specification, and therefore the stable operation of the device can’t be guaranteed.