Choose the weight up to which you've proven capable for a particular exercise movement.
When under tension, the Trainer+ starts at 8 lbs (4 kg) and can be adjusted upwards as it learns how strong you really are.
Once you have proven to the machine (via the Strength Assessment) that you can lift up to a specific weight, it will not let you select more than that weight as you have never lifted it before.
This is a safety feature to ensure that nobody accidentally lifts more than they have ever done.
If there is no known data of your strength for a particular exercise, the app will cap the load at 44 lbs (20 kg) for your safety till you unlock it in a class, or the Strength Assessment.