On Lewis Hamilton's 2015 steering wheel, the thumb wheels labeled ENTRY and HI SPEED (1 and 2) are linked to the differential, and are used for turn entry or for a high speed corner.
The MID and INIT wheels (3 and 4) are also linked to diff settings, although their precise function is not known. While the MID label probably commands settings associated with a mid-speed corner, the INIT denomination is less clear as it appears to replace the former EXIT function used upon the exit of a corner. On the W06, the wheels no longer appear symmetrical like on last year's model (where ENTRY was positioned right next to EXIT, and MID next to HI SPEED).
Their prominent positioning obviously indicates the importance of these wheels: the driver must be able to swiftly change the parameters at any time during the race, according to tyre degradation, fuel consumption, overall race strategy, etc...