Where to place lowpoly object compared to highpoly object? I am confused, when I want to bake normal map from high poly object I need to duplicate it and make the copy the low poly object I want then 
1- One tutorial I saw said that I need to place the low poly is the same place of the high poly.
2- Other tutorial said I must give it higher position on the z axis than the high poly.
So which one is it?
Normaly, the lowpoly is in the exact same place as the highpoly object. But if the lowpoly is completely flat, it could be "above" the highpoly. Look at this answer where I project the normals upwards with baking.

Creating game models for android

If your lowpoly object is not flat, you can't move it if you want to bake normals.

If you are willing to bake the lowpoly piece by piece, and each piece is relatively flat you can move details from the highpoly object. Then you must fiddle with the offset parameter and this is usually a pain to get right.