The difference is for machines that can pre-stage tools.
"T on M6 line is next tool":
T1 (Tool changer stages T1)
M6 T2 (Load T1 into the spindle and stage T2)
...
M6 T5 (Load T1 into the spindle and stage T5)
"T on M6 line is tool to use":
T1
M6
...
or
M6 T1
...