Protocols and standards
MH, MR and MMR
MH, MR and MMR are the three standard compression methods for a fax image, from the simplest to the most efficient.
MH, Modified Huffman, codes each line on its own, replacing runs of white or black dots with short codes. MR, Modified READ, codes some lines as a difference from the previous one. MMR applies that principle to every line and saves even more space, but a single error spreads through the rest of the page.
Why it matters for fax
That is why MMR is only allowed in Group 3 with error correction: without it, one burst of noise would destroy half a page instead of one line. The two machines agree on compression during negotiation, and that choice directly affects transmission time.