Hi @Shaned72 ,
About 99.9% of the time, you’ll want to be in AUTO mode. Most of the time, AUTO functions like ERG mode in other platforms, where the app & trainer will control the resistance for you. However, any workouts that say MIXEDMODE in the description means that there will be interval(s) in the workout done in SLOPE mode (at a pre-defined slope %). You’ll also use AUTO for these - Xert EBC will automatically switch from ERG mode to SLOPE and back without any manual intervention.
Most of the MIXEDMODE workouts are either:
- Sprint workouts with all-out 10-20s sprints, or
- Workouts designed to be performed all-out, such as the micro-intervals found in Ronnestads/Billat workouts
I’d encourage you to try one out. It will probably take some time to get ‘used’ to it & figure out the gearing/cadence you need to hit the interval targets, but very effective once you understand how they work 
Cheers!