With -m, incomplete lines (see Incomplete Lines) are simply copied to the output as they are found; if the merged output ends in an conflict and one of the input files ends in an incomplete line, succeeding ‘|||||||’, ‘=======’ or ‘>>>>>>>’ brackets appear somewhere other than the start of a line because they are appended to the incomplete line.
Without -m, if an ed script option is specified and an incomplete line is found, diff3 generates a warning and acts as if a newline had been present.