The _BIT datatype can hold only 0 and -1. If you set the variable to any other number then the least significant bit of that number will be set as the variables number, if the bit is 1 (on) then the variable will be -1 and if the bit is 0 (off) then the variable will be 0.
A _UNSIGNED _BIT can hold 0 or 1 instead of 0 and -1, the rest is the same.
The least significant bit is the last bit on a string of bits (11111) since that bit will only add 1 to the value if set. The most significant bit is the first bit on a string of bits and changes the value more dramatically (significantly) if set on or off.
The _BIT datatype can be succesfully used as a boolean (TRUE or FALSE) and it requires minimal amount of memory (the lowest amount possible actually, one byte can hold 8 bits).