Setting the policy to Enabled isolates all sites. (Each site runs in its own process.) Setting the policy to Disabled or leaving it unset doesn't turn off site isolation, but it lets users opt out (for example, by using Disable site isolation in chrome://flags).
IsolateOrigins might also be useful for fine-tuning origins. On Google Chrome OS version 76 and earlier, set the DeviceLoginScreenSitePerProcess device policy to the same value. (Because if the values don't match, a delay can occur when entering a user session.)
Note: For Android, use the SitePerProcessAndroid policy instead.