The family templates contain shared parameters like length, height, ect. that to my understanding are not accessible to the public. The typical solution would to make a separate shared parameter with the same name, which leads to confusion and further fractures the usefulness of the "Shared Parameter" feature.
Since labels already maintain a "system" list of shared parameters, it is strange to me that I have not found any way for custom families to load them. This would dramatically improve the standardization of user-created models, and benefit the community.