On July 1, 2012, the American Kennel Club (AKC) added a few more titles for dogs and their handlers competing in agility to reward them for their longevity at the Master levels of competition. These new titles are retro active for dogs competing after 1999.
From the AKC agility rulebook (located here):
Chapter 2, Classes, Titles, and Height Divisions
Section 3. Recognized Achievement. In order to recognize consistency and longevity in the Master Agility Excellent (MX), Master Excellent Jumpers With Weaves (MXJ) and Master Excellent FAST (MXF) classes, the AKC will record a Lifetime Achievement title on the dog’s record. There is no limit to this achievement. (For example, a dog with the MXC title plus twenty-five additional Excellent B Agility class qualifying scores would have the MXB2 title.)
The AKC will identify dogs qualifying for Lifetime Achievement titles by the appropriate title designations (MXB, MXB2; MXS, MXS2; MSG, MSG2; MXC, MXC2, etc.) following their AKC name in all official AKC records. In each case, the higher Lifetime Achievement title will supersede the preceding Lifetime Achievement title in all official AKC records such that the highest title will be listed.
The Lifetime Achievement Standard titles and requirements are:
Master Bronze Agility Title = MXB. Requires twenty-five Excellent B Agility class qualifying scores of 100.
Master Silver Agility Title = MXS. Requires the MXB title plus twenty-five additional Excellent B Agility class qualifying scores of 100.
Master Gold Agility Title = MXG. Requires the MXS title plus twenty-five additional Excellent B Agility class qualifying scores of 100.
Master Century Agility Title = MXC. Requires the MXG title plus twenty-five additional Excellent B Agility class qualifying scores of 100.
The Lifetime Achievement Jumpers With Weaves titles and requirements are:
Master Bronze Jumpers With Weaves Title = MJB. Requires twenty-five Excellent B Jumpers With Weaves class qualifying scores of 100.
Master Silver Jumpers With Weaves Title = MJS. Requires the MJB title plus twenty-five additional Excellent B Jumpers With Weaves class qualifying scores of 100.
Master Gold Jumpers With Weaves Title = MJG. Requires the MJS title plus twenty-five additional Excellent B Jumpers With Weaves class qualifying scores of 100.
Master Century Jumpers With Weaves Title = MJC. Requires the MJG title plus twenty-five additional Excellent B Jumpers With Weaves class qualifying scores of 100.