According to the tutorial, below is the rules for Batch.
- Have the same Batch Group ID
- Have the same material
- Have the same shader parameters
- Be within a bounding box with no side larger than the Max AABB Size
- Be in the same layer
- Each batch has a maximum vertex count of 65535
- For dynamic batches there is a maximum number of movable mesh instances. This hardware dependent but has a maximum of 1024.
I just wonder if mesh objects with different shape can be grouped as the same batch group.