This means that transfer wraps within the same line and column. i.e. if drawing in reverse and overlapping x=0, then pixels will wrap to 256-x on the same line and if overlapping x=256 then pixels will wrap to 0.
=== LMMV ===
* In bitmap mode, R48 is used for VRAM0 and R49 is used for VRAM1. For example, if R48 is set to #34 and R49 is set to #12, and 8bpp mode is used, x=0 will be set to #34, x=1 will be set to #12, x=2 will be set to #34 etc. This is why the documentation states that for 8bpp, 4bpp and 2bpp modes you should repeat the pixels through R48 and R49 to ensure a consistent fill value.