Does each week mean that a DVC member is not staying at their home resort - and they have made plans for that week elesewhere (another DVC resort, cruise, RCI exchange).
Yes and no. These deposits are for those Members who have exchanged their points for something in the World Passport/RCI collection. Members who stay at another DVC resort just use their points to book that room, and that has no effect on external availability. When Members exchange points for a cruise, the cruise is "paid for" by booking a room for that many points, and giving it to CRO to try to rent out, and the proceeds offset the cost of the cruise.
So, these deposits are all a result of Members either (a) exchanging points for a confirmed weekly reservation somewhere in RCI, or (b) depositing points for a "credit" towards a future exchange.
I have dev points with Saratoga Springs that I know I can bank into RCI and have them for use for like 2 years. Does this mean that I may at some point be able to use those "banked dev points" for a DVC stay at any of the resorts that are listed in the first 2 messages of this thread?
No. You can only use your "banked" points to exchange into one of the "approved" RCI resorts. I don't believe any Orlando resorts are on that list, including the DVC resorts. This was true in II as well.
Or, possibly, DVC members services holds them as members bank them , then they all go in together at the start of each month?
It appears that this is exactly what is happening, though typically resort systems bank in advance of confirmed exchanges, rather than in arrears, but RCI is also willing to extend "credit". This is a pretty common arrangement---Wyndham has the same setup. Wyndham owners make exchanges throughout the year, for which Wyndahm has deposited some in advance. Periodically, RCI and Wyndham "balance the books", and Wyndham increases or decreases its deposit rate based on exchange activity in the prior period. If the DVC/RCI relationship works the same way (and it appears that it does), DVC Members appear to be more active in RCI than they were expected to be---this is a much bigger deposit than we saw in January.
How does banking points work anyway? Do we select a week then bank it into RCI? Or do we tell members services (we don't want the points) and they select what (resort, week, etc...) gets banked?
You relinquish a set number of points. DVC chooses how to use those points to deposit units on the Members' behalf. I don't know that anyone knows exactly how that decision is made---Dean probably knows more than most. Convetional wisdom is that DVC tries to deposit less-demanded resorts, times, and units, but there are some prime things that get deposited as well.