Minecraft wiki entry for Development_versions, associated text: Added a new line detailing mob spawning.
SC represents the number of chunks in which mobs can spawn. It is the same value given in debug reports as spawning_chunks.
M, C, A, W, and M represent the number of mobs for each group that contribute to that group's mob cap. The groups are Monster, Creature, Ambient, Water_Creature, and Misc. They're also in debug reports.
In the Targeted Block and Targeted Fluid fields, the coordinates of the targeted block or fluid are now shown as well.
The "Looking At" field has been removed in favor of this.