So how do you know what you can write or read from the SLAVE device? These are most likely documented in the devices data sheets (it pays to read the data sheet carefully).
For example we want to interface with a simple Dallas DS1307 (64x8 Serial Real-Tine Clock). On page 4 of the data sheet, we find the address or register map. These addresses contain the memory locations that we can write to and/or read from.
These registers are valid only for that specific device. Different devices will likely have different registers.