v  may be any integer variable or a system variable such as relay[n], output[n], etc. However, if v is a 32-bit integer, SETBIT will only operate on the lower 16 bits. The range of n  depends on the PLC firmware version, as follow:

n is an integer constant or variable of value between 0 and 15 or between 0 and 31. Following digital electronics convention, bit 0 refers to the least significant bit (rightmost bit) and bit 15 the most significant bit (leftmost bit) of a 16-bit variable and bit 31 is the most significant bit of a 32-bit integer variable.

For older PLC firmware (before r78), CLRBIT only works on the lower 16 bits.