@Talent312 mentions some affiliates being HGVC enrollment being optional. I wanted to add that there are some affiliates in which HGVC enrollment only comes from a retail purchase. Grand Pacific Palisades (GPP) and Grand Pacific Seapointe are two examples.
I purchased my GPP from the resale department at the resort. At the time, GPP handled all resales for existing owners at decent resale prices. As a bonus, they added HGVC enrollment at no extra charge. This was a true benefit for owners looking to sell and I am confident that it helped owners maximize value when selling. As HGVC got more involved, they apparently did not like this cheaper option into HGVC and stopped it from happening. GPP no longer has a resale department that helped the owners. Now, HGVC handles the deed-backs and foreclosures and turns them into over priced retail deeds. In addition, they made it so the HGVC enrollment disappears when the original buyer decides to sell the deed hurting their resale price. What that means is that the only way to get HGVC enrollment on these deeds is through a direct purchase.
There are so many nuances, evident by the many posts, that it is best to slow down and research until your head hurts. The developers like to make one believe that the retail deal they are offering is only good today and prices will only go up. With resale, the opposite is true. There will always be resale deals. In fact, the deal tomorrow may be better than the deal today.