8TF refers to those 1878-P Morgan Dollars in which the eagle is depicted with eight tailfeathers.
A revision to this 8TF reverse called the 7/8TF variety appears on some 1878P coins as an intermediate
conversion to the 7T tailfeather (7TF) design which persisted throughout the rest of the Morgan Dollar production.
You will infrequently hear VAMmers refer to this reverse as the type "A" reverse, which is its technical name. The type "B" and "C" reverses are known as the Reverses of 1878 and 1879, but these reverses only have 7 tailfeathers, hence the 8 tailfeather distinction.
The 8 tailfeather reverse was the first used in the Morgan series, and the first exact dies used were for the 1878-P VAM-9. There are a few dozen different 8 tailfeather varieties, and many VAMmers enjoy collecting them. There are many 8 tailfeather varieties that are ultra rare, making completing an 8tf set an extraordinarily difficult task in any condition. Please click this link for a list of eight tailfeather varieties.
See the photo below for an example of a eight tailfeather (8TF) reverse. The eight yellow lines point to the "eight tailfeathers", while the two red arrows point to the areas of what are called "engraved feathers". The engraved feathers were cut into the die after the die was created and are key to identifying the reverses of 1878 eight tailfeather varieties. Attribution Guide for 1878-P Morgans Attributing 1878-P VAMs
This is the reverse of the 1878-P VAM-15, a Top 100 VAM variety (also the reverse of the 1878-P VAM-10, which is not as rare as the VAM-15).
Please click this link for the full VAMworld.com definitions page.