The window refers to the earliest point before departure that AA will process upgrades using "segment" upgrades, not when you can "get in line." All elite members (Gold, Platinum, and Executive Platinum) can request "segment" upgrades at the time of booking. Requesting an upgrade adds you to the upgrade list for your flights ("get in line"). The earlier you request the upgrade, the earlier on the list you are. Executive Platinum members, and all elite members on flights 500 miles or under, are auto-requested, but Gold and Platinum members need to manually request upgrades (by clicking the box next to the flight when you view your itinerary on aa.com).
Keep in mind that for flights over 500 miles, Platinum and Gold members need to spend upgrade "segments" for upgrades. You need one "segment" for each 500 miles or fraction of the flight. So, a 1,200 mile flight costs three "segments." You earn "segments" as you fly, and can buy additional ones online or from an agent (they are cheaper online).
Depending on how a flight is selling (and other factors such as weather issues), AA's Revenue Management (RM) group determines how many seats are available for "segment" upgrades. This number is secret, you can't see it, but it's there and is constantly adjusted over time as conditions change.
At 100 hours before the flight, AA starts processing upgrades for Executive Platinum (EXP) members. If RM allocated 3 seats for upgrade, then the first three EXP members on the list get upgraded. There might be fewer EXP members than upgrade seats, so some seats may remain, and additional seats might be allocated within the 100-hour window. AA continues processing upgrades right up until a few hours before the flights. At 72 hours before flight, Platinum (PLT) members are eligible, as well as EXP. At 24 hours, all elite members are eligible.
When AA processes upgrades, the list is ordered first by status (all EXPs, then all PLT, then all GLD). Within each status level, it is ordered by full-fare (Y and B codes) vs discount fare (all other coach fare codes), and then by time requested. So, booking early and requesting immediately give you a better chance.
Because Platinum and Gold members need to spend "segments" to get upgraded, most will pick and choose which flights they most care about the upgrades, and request only those. That means that the odds of getting upgraded as a Platinum depend on how many EXP members are on your flight, and how many PLTs are on your flight who have requested upgrades and did so before you did (and if any of them bought Y/B fares, which are not that common).
Once the flight goes to airport control a few hours before departure, AA stops processing upgrades, and everyone who is still waiting for an upgrade gets moved to the airport upgrade list when they check in. You can see your position in this list by using the aa mobile app, or at the airport on the gate monitor. Note that people don't get added to the list until they check in, but when they do check in, their position on the list is based on where they were before, not when they checked in.
If you want the best shot at an upgrade, fly on days and times that are not popular with business travelers (who are most likely to be EXP), such as Saturdays, and to a lesser extent, late morning through mid-afternoon, buy your ticket far in advance, and request the upgrade right away.