Set Bit enables you to select a bit within a register, and set it.
Reset Bit enables you to select a bit within a register, and reset it.
Operand A, Start of Vector, is the register in which the function will set/rest the bit.
Operand B, Offset in Vector, selects the bit within that vector.
Note that the maximum number of bits in the vector is 255, 16 MIs or 8 double registers. The functions are located under the Logic menu on the Ladder toolbar.