“Main mass” is a scientific term used to describe the largest single extant piece of a specific meteorite. For example, the main mass of Clarendon (c), found in Texas in 2015, is a whopping 345 kg, but smaller pieces were also recovered and all of those are believed to have weathered off from it. We often think of a main mass as being very large, but a main mass can be of almost any size, depending on how much of that particular meteorite was recovered in the field and how much remains intact today.
When a new meteorite is discovered and submitted for classification, a sample (typically about 20 grams) is removed and sent to the institution carrying out the analysis. That removed sample becomes known as the “type specimen” and it remains permanently with the classifying institution. If no other meteorites are determined to be a match to the new discovery, the remaining and larger portion becomes known as the main mass. In the majority of cases, we would expect that mass to be cut up into smaller pieces and sold, traded, or sent to additional institutions for further study.
In rare instances, the larger piece may be kept whole and made available for sale to collectors as the official main mass. As with this nearly a kilo stone meteorite.