Hi @Anonymous
The Placement Plane defines where the rebar is placed in the host.
If you have a structural element cut by a section view and you are placing the bar in that section view, you will get the following results when using the 3 placement plane options (depending on the view direction, the farthest face of the element, the current cut (work) plane, or the one "closest to the screen")

More about placement here
and rebar shapes here
Furthermore, each rebar set has constraints that make it react to changes of the host.
Bar plane - the plane where all the curves of the first bar in the set are located
Out of plane extent - the plane containing the curves of the last bar in the set (to where the set extends)
Bar segment 1-n planes - the plane of each bar segment - this is perpendicular to the bar plane
Bar start / End - where the start and end of the bar is positioned
You can edit the offset and face of each handle by using Edit Constraints.
Check out the attachment for a video about constraints.
The Placement Orientation defines how the rebar is aligned when placed in the host.
It's how the bar plane is positioned when you first place the bar.
The same example using some of these options:

My advice for you is to play around with these options and see which one works best in a given situation.
Let me know if you have any questions.
Ovidiu Paunescu, M.Sc. Str. Eng.
Sr. Product Owner | Autodesk Revit