As I found out recently this is a name for unborn or still born calf skin. The hair is shorter then regular calf skin, and it is very slippery and yet tough. I suppose that this is nature (God's) way of making birthing as easy as possible.
I was told about this by someone with a lot of history in archery, and a very good archer. I hesitate to use his name at this point but many of you would know him. My initial opinion of this material for tabs is "Wow, where has this been." As for wear, I was told that it lasts a long time. I have only used my tab for about 200 shots now so it's too soon to detect any wear.