I'm an owner of Bay Club without joining HGVC (personal preference). Here are the limitations/rules.
You have an open period of week 1-50 (mostly) Weeks 51 and 52 were sold separately as extra value weeks. Your booking window starts in December, the year before. For example the 2019 booking window started last month. It is done on paper, and not online. (You are sent a paper form to fill out.) The entire window is open for booking at that time from week 1 to week 50. You will get notification back in a couple of months. (They juggle the pile of paper to try to get everybody what they want.) I usually go for January, as February is usually too high of a demand for my needs.
You can only book the type of room you purchased. There are 9 types. (There is a attachment with them somewhere on TUG but I have lost the link - I downloaded the file to keep handy a while back). So if you purchased an "A" Penthouse, you can only reserve an "A" Penthouse, and no other type.)
A. UNIT TYPES. The Units in this Program are currently divided into nine (9) types, as follows:
TYPE I UNITS. Deluxe one bedroom, with sleeping accommodations for up to four (4) persons.
TYPE II UNITS. Deluxe two bedrooms, with sleeping accommodations for up to six (6) persons.
TYPE III UNITS. Penthouse one bedroom, with sleeping accommodations for up to four (4) persons.
TYPE IV UNITS. Penthouse two bedrooms, with sleeping accommodations for up to six (6) persons.
TYPE V UNITS. Grand Villa two bedrooms, with sleeping accommodations for up to six (6) persons.
TYPE VI UNITS. Deluxe two bedrooms, A floor plan, with sleeping accommodations for up to six (6) persons.
TYPE VII UNITS. Deluxe two bedrooms, B floor plan, with sleeping accommodations for up to six (6) persons.
TYPE VIII UNITS. Penthouse two bedrooms, A floor plan, with sleeping accommodations for up to six (6) persons.
TYPE IX UNITS. Penthouse two bedrooms, B floor plan, with sleeping accommodations for up to six (6) persons.
In addition, you can only book a week at a time. No individual days.
But it doesn't stop there! Weeks cannot be block booked. Each week is booked separately (its own piece of paper). You can request that your ressies be blocked together, and so far I haven't had a problem getting them together in January, but the booking process is different than Marriott's multi week process.
Finally, each room is assign a day of the week for it's start/stop period. Some start on Thursday, some Friday, some Saturday, and some Sunday. If you want a specific room, you have to adjust your stay period to match the room's availability. No flexibility on room/day of week assignment.
Those are the limitations. The advantage is you get first crack at the inventory, ahead of ALL points users. And you don't have to pay any extra fees, other than re-reservation fees if you are inside the 6 month use window.
So, if this is what you want, (and for me it is), you can't beat the space/cost MF ratio. This year I paid $1553 a week for an "A' penthouse. 1900+ sq feet (including the lanai). No ocean view on any of them, (except a distant peek-a-boo on some units), but I don't want to pay for a view. Just warm winds and peace. . . .