The PDAF is the Phase Detect Auto Focus. This the fast AF and it’s located in the middle of the sensor. The larger squares in the selection matrix shows the PDAF capable cells. The smaller squares near the edges are the CDAF (contrast detect autofocus) cells.
The phase-detect AF points need a vertical edge to focus on. If there is no vertical edge, there is no focusing in PDAF and the camera reverts to CDAF.
Vertical means that if you are using the camera sideways/vertical/portrait orientation, then the PDAF will look for horizontal edges.
The PDAF focusing cells are also able to switch to CDAF.
Vertical means a kind of vertical direction, so in my experience a 33° is vertical enough.