If you consult wikipedia about the 'Ultra-Trail du Mont-Blanc' (UTMB), here is the intro you will get:
Since UTMB Group's partnership with IRONMAN Group in 2021, UTMB World Series will formally launch in Year 2022, and Ultra-Trail du Mont-Blanc will be renamed as UTMB World Series Finals from Year 2023 Edition, as the annual final competition of this series.
UTMB is easily the most influential factor in the world of ultra-trail running. As can be deduced from the wikipedia intro above, UTMB is both an organization (the 'UTMB Group'), a race (simply the 'UTMB') and a world encompassing system of ultra-trail races (the 'UTMB World Series').
If you look at the headers for the MMC 21K, 42K, 70K and 100K distances, you see that they have associated UTMB Distance Categories like "UTMB 20K", "UTMB 50K", and "UTMB 100K". This is because the MMC 20K, 42K, 70K and 100K distances are approved and classified by UTMB as UTMB Index races. To quote UTMB:
From 2022 onwards we are using a new system, races will no longer be awarded 'Qualifying Points'. All races will now be placed in to 1 of the 4 UTMB Distance Categories, which are 20K, 50K, 100K and 100M.
Participants who finish your race will receive a UTMB Performance Index in the category of the race. The Performance Index is used in a similar way as the ‘qualifying points’ to show a runner’s experience at different distances and can be used to enter the UTMB World Series Events and combined with Running Stones to enter the UTMB Finals.
The UTMB Performance Index is very similar to the ITRA performance index, if you are familiar with that. In fact, if you have completed one or more trail races in the future, there is a high likelihood that you will be able to find your own ITRA performance index by visitin the itra.run web site. Similarly you can visit https://utmb.world/utmb-world-series/runners/utmb-index to check if you already have a UTMB performance index. Here is an example of what you might find:
To the left is the general UTMB index. This reflects the general performance level of the runner, and is based on all the UTMB approved races the runner has completed in recent years. Then the indiidual performance indices for each of the four distance categories are shown. In the example above, the runner has established performance indices for the 50K and the 100K categories, while the runner has not completed enough 20K and 100M races (recently) to get specific performance indices for those categories.
For many ultra-trail runners, the UTMB and sibling races represent the holy grail f ultra-trail running. If it is on your bucket list to do one of these iconic races, the MMC will contribute to give you a valid UTMB performance index. In addition you will need UTMB running stones, which you can gather in any of the UTMB World Series events.