I do hope your document has some pictures in it - a document is worth a thousand words, they say... :-)

Both of these terms CAN mean the same: the wooden (PVC plastic, etc.) blocks forming the frame around the window. This casing/frame is fixed to the wall when windows are installed. On the inside this casing/frame has hinges that support the "actual" windows - rectangular frames with handles that open and let the air in (or do not open, like in New York skyscrapers).

Meaning of words is defined by the context, so stating that "these two terms are ABSOLUTELY and ALWAYS the same" is unwise.

For example:

"Window casing" may mean:
1) window frame
2) the lining around the window - NOT part of the frame.
3) window aperture - the "hole in the wall" in which window is located.

It is highly likely your text deals with meaning #3 of "window casing" for the following reasons:

"Internal installation – within the window casing" - you cannot install a roller shade INSIDE the window (as far as I know), so we are talking about installing it inside this "indentation in the wall" with beveled edges that leads to the window.

"A window casing depth with at least 114mm (4.5”)" I personally cannot imagine window frame 11.5 cm thick. Windows (even here, in Russia, where it is sometimes very cold) are thinner than that - about 5, maximum 8 cm. Here wer are talking about thin walls and shallow "hole in the wall" measured from window surface to surface of the wall.

"A shallow window casing depth" - I guess it is the same as above.


Personally, I would contact the client (through the agency if required) and ask for clarification by giving the options above and asking for the right term (or for their brochure etc.) This is not a sign of professional weakness - to the contrary, wise clients understand this to be a sign of maturity and diligent approach to translation.

Good luck!