As you may have guessed, trading can range from anywhere between being simple to complex.
Variables include the Resort itself, the 'Value' of the time (Often called red time, etc), the type of program (fixed week, points), and the type of unit (1-B/R, 2-B/R), and whether the unit itself is fixed or variable.
In it's simplest terms, you own at "A", someone else owns at "B". You want to go and stay there, they want to stay at yours. You trade, done deal.
But in reality, trading is more of a round robin. You own "A" but want to go to "B". Someone in "B" wants to go to "C", someone in "C" wants to go to "D", and someone in "D" wants to go to "A". A complete loop is found and all those trades can take place. Now, instead of "A" through "D", imagine you have resorts "A" through "ZZ". That's a lot of combinations. It's almost like you "have a friend who's brother works with someone who is dating a person who's father works at....." and so on.
And it's dozens, or hundreds of potential loops.
The computers try to find the combination that gets you your trade, but also has everyone else get the trade they want.
Now the reality of it all: You want to go to Hawaii. The only way that can happen is if someone there trades OUT. So who gets that slot? The program's first priority may be to it's own members, such as at Marriott. Maybe there's 20 Marriott owners wanting to trade into Hawaii, They all come first before anyone else. If only one person trades out, then 19 of those Marriott owners don't even get in, never mind anyone else.
Now assume 21 Hawaii owners trade out. Those 20 Marriott owners get the 1st 20 spots, leaving only one left. That one is going to II.
DVC owners would be among those near the top of the "desirability" list. But if there were 10 DVC owners, maybe 1 would get it, maybe it would go somewhere else, maybe a high end Hilton owner somewhere.
In any case you can imagine how far down the list someone would be who owned a $6000 timeshare at Daytona Beach for example, trying to get that Marriott Hawaii week.
Likewise, trading INTO DVC is similar. Someone at DVC has to trade out. But they probably don't want to go to that Daytona Beach unit. They want something better, and state it's item "A" or no trade at all. So the loop search begins. Someone in Hawaii, someone at South Seas plantation on Sanibel Island, and so on.
Maybe an anology will work: 100 people drive to this huge parking lot, and they all agree to swap cars. The BMW owner sees a Mercedes and swaps, but then he sees a Caddy and swaps for that. Meanwhile the Oldsmobile owner saw a nice Pontiac, and the Ford guy traded for a nice Mercury. That Mustang GT owner found an older, but nice Corvette.
Meanwhile, you brought your 1988 Honda. When all the trading is done, what chance is there that you'll wind up getting that 2002 Mercedes?
You'd be asking for this scenario: The Mercedes guy wanted the Cadillac, the Caddy owner wanted that Oldsmobile, the Olds guy wanted that older Corvette, the Vette owner wanted that classic Mustang, and the Mustang owner wanted a 1988 Honda. Basically EVERYONE in the loop had to trade down a little, and you got all of that by one big trade UP. Possible: Yes, Probable, NO. Remotely probably: NO
Does that make sense at all? Hope maybe this helps.