Question about Z-axis

When Exporting .obj , z-axis will be the vertical one … however, I saw many cases, the Vertical one is not z-axis ( Most are Y and X axis…).

May anyone tell me the reason why ? thank you

There is no standard over coordinates system, this table shows the different systems used by various software as an example. Note how Maya and 3DS Max don’t use the same system despite being produced by the same company.

Most 3D file format don’t define where is ‘up’ or where is ‘left’, they only take your list of vertices without knowing in what coordinates system it should be mapped. Therefore, when you load for instance an FBX file in three.js, three.js has no way to know if it’s supposed to adapt the vertices to its own coordinates system ( right-handed, Y is up ).