"Standard expectations" for a renderer

[BePie]

the documentation of QQuickFrameBuffer::mirrorVertically mentions "standard expectations" of a renderer:

This property controls if the size of the FBO's contents should be mirrored vertically when drawing. This alows easy integration of third-party rendering code that does not follow the standard expectations.

What are the "standard expectations"?

are those:
x: from left to right
y: from top to bottom
and what about z? If z is from screen (so that z-axis in QtQuick matches) than the coordinate system is "left-hand" and I don't see this as a "standard" (I thought right-hand side cs is the standard). Where I can find more about the topic?