A block family is a group, or family (hence the name) of blocks. What this means is there are an arbitrary number of blocks that are grouped together because they are similar. For example, there is a RomanColumn family which consists of a cap block, a base block and a middle block. These blocks are separate blocks with different textures, and they can also have distinct shapes (although RomanColumns are all cubes), but they share a *.block file. A block family is created with the "rotation" tag in v1 block families and the "family" tag in v2 block families. For more information about v1 block families and v2 block families, look here. This gist aims to compare and contrast the block family versions and add some documentation as well.
What I'm referring to as old block families are the current block families in the master Terasology repo as of the time of writing (Friday, December 1 2017). [Example of ol