Rolls: Client side or server side?

[Rongway]

Server side on a first come first serve method.

Unless someone else can prove it wrong, the moment someone "rolls" they generate the number which means the next 3-7 people who roll after that, cannot generate the same number (which is why we never have duplicate rolls).

That, or the moment an item drops, the game calculates a hidden number for us that we only see when we roll, but it was already previously chosen for us.

A good example would be when I hit "need" on an item in maps and got a 99, but they were still discussing if we should split the take (first day Elpis maps) and the others were like "ok, ok, roll!" but I already knew all their rolls were pointless, because I hit the 99 the moment it popped up.

Random number requests don't directly use the number generated by the generator. Whenever the generator gets a rand request it generates a number probably in the range
[0..4,294,967,296] (2^32 - 1)
or
[0..18,446,744,073,709,551,615] (2^64 - 1)
and this number must then be mapped to some smaller range depending on the use case, such as [1..99] for loot rolls or [1..some power of 10] for action proc rolls. Because there are more than 99 possible raw rand values, in the case of loot rolls the Pigeonhole Principle guarantees that there are multiple values that will map to the same result value. Therefore, it is absolutely possible for two sequential rand requests from the generator to produce the same result.

However, it's unlikely that you'll ever see two consecutive rand requests from the generator because of the sheer volume of requests that the generator serves. Even were your entire party to roll in the same 0.1s timepan, it's quite probable that other people have also triggered rand requests between the times each of your party members are served their numbers.