Crab Clusters (Half-Crab Sections)
Crab clusters are popular for serving at crab dinners because they are attractive. Frozen crab clusters also tend to maintain quality in the freezer better than individual legs do. The shell of the shoulder helps to protect the delicate crab meat from damage by the dry freezer air.
The white cluster or "shoulder" section comes from underneath the main body shell (or 'carapace') of the crab. When crab are processed, they are broken in half, and the result is... 2 crab clusters.