|const unsigned int DirectAccessBit = 0x40|
Means that the underlying array of coefficients can be directly accessed as a plain strided array. The memory layout of the array of coefficients must be exactly the natural one suggested by rows(), cols(), outerStride(), innerStride(), and the RowMajorBit. This rules out expressions such as Diagonal, whose coefficients, though referencable, do not have such a regular memory layout.
See the comment on LvalueBit for an explanation of how LvalueBit and DirectAccessBit are mutually orthogonal.