'reserved for future' bits in microcontrollers
Unfortunately your questions require one to be psychic to answer them, as not even the manufacturer may know yet. But it is very easy to state one crystal clear thing about them: do not use them!


  The original Macintoshes shipped with the 68000 microprocessor. While this
  CPU is capable of doing 32 bit operations, it has only 24 bit addressing
  capability.


[...]


  Back when the Macintosh only
  had a 128k of RAM, the Operating System had to go to some extreme lengths
  to ensure that application have enough memory to run. The Macintosh Memory
  manager allows blocks of memory to move, and/or be purged if the System is
  having trouble fulfilling a memory request. The original designers of the
  Macintosh OS decided to use the last three of the unused bits in a 32 bit
  memory address to indicate whether a block of memory can move, be purged or
  if the block contains a resource item.


[...]


  The problem is that to set these three bits,
  the Operating System routines have to call other routines who have to call
  still others etc. The net result is that using the Operating System
  routines to set these bits is quite inefficient when it comes to speed.
  Therefore, prior the introduction of System 7, some creative programmers
  with a need for speed, took it upon themselves to set these bits in the
  memory block's addresses directly thereby bypassing the overhead associated
  with calling the Operating System routines. Of course, the problem with
  doing this is that System 7 no longer stores these three bits in the
  address of the block of memory. Another significant programming error
  involves the other 5 bits of the 32 bit address. Ordinarily, these bits
  should remain unused and therefore, insignificant. However, some
  programmers, having realized that 5 bits are wasted decided to use them for
  their own purposes, even though Apple Developer Technical Support began
  warning them against this practice a full three years prior to the
  introduction of System 7.


[...]


  The fundamental problem with setting the upper 8 bits of the address
  directly is that with System 7 all 32 bits of information are used for
  addressing. Changing the value of any of the 8 bits changes the address of
  the block of memory. When an application or an init tries to access the
  block of memory that now has an invalid address, the usual result is a Type
  1 error. This occurs because the first 24 bits of an address are used to
  access memory locations between 0 and 16mb. The upper eight bits are used
  to access memory locations between 16mb and 4,096mb. Since most
  Macintoshes have less than 16mb of RAM, chances are this incorrect memory
  location is pointing to an address that does not physically exist, and this
  will yield a Type 1 (Bus Error).
  
  In the event that the memory location does physically exists, then the
  application or init will then operate on whatever information it finds at
  the incorrect location. Depending on what the application or init is
  attempting to do, various errors may result.