ZeniMax is calling the raid a “choose-your-own-adventure style of Trial” because you can take it on in the order you choose and in combination with mode selection thereby set your own level of difficulty.
“The Asylum Sanctorium is a new type of Trial for both veteran and rookie groups,” the studio says. “Like previous Trials in The Elder Scrolls Online, this new challenge requires 12-players and a high level of coordination and communication in order to complete. However, this new challenge also has two big differences: It only has three bosses total and allows you to pick how you want to take them on.”
Source: Official site