compression - What does the number after 7-zip's -m switch mean?

7-zip has a command line switch to set the compression method, -m followed by a number, e.g.
-m0=LZMA.

What does the number (0 in the example) mean? Different numbers produce slightly different compression results and performance:

Parameters                      Wall    User    System
-m0=LZMA -mx=9 -ms=on -mmt=off  28.4    27.48   0.85
-m1=LZMA -mx=9 -ms=on -mmt=off  27.45   33.06   0.90
-m0=LZMA -mx=9 -ms=on -mmt=on   12.74   24.39   1.14
-m1=LZMA -mx=9 -ms=on -mmt=on   15.08   33.14   1.28
-m0=LZMA -mx=9 -ms=on -mmt=off  26.5    25.58   0.65
-m1=LZMA -mx=9 -ms=on -mmt=off  27.07   32.84   0.87
-m0=LZMA -mx=9 -ms=on -mmt=on   13.27   24.99   1.00
-m1=LZMA -mx=9 -ms=on -mmt=on   15.32   33.28   1.47