The BITOP command supports four bitwise operations: AND, OR, XOR and NOT, thus the valid forms to call the command are:
BITOP AND destkey srckey1 srckey2 srckey3 ... srckeyN
BITOP OR destkey srckey1 srckey2 srckey3 ... srckeyN
BITOP XOR destkey srckey1 srckey2 srckey3 ... srckeyN
BITOP NOT destkey srckey
As you can see NOT is special as it only takes an input key, because it performs inversion of bits so it only makes sense as an unary operator.
The result of the operation is always stored at destkey.