When a poster is present in the given image, but the uploaded image is not a poster. This is usually pasted/hanged on the wall. (See post #10648, post #38736)
Real-world posters that were photographed where parts of the room are still visible fall here and go with the photo tag. (See post #19090)
Use poster if the entire image by itself is a poster. (See post #8686)
In some cases, an image can be tagged both poster and poster_(object). (See post #775810)