A BT (breakthrough effort) is all about MPA drawdown and proving your mettle under fatigue or in the case of PP (peak power) demonstrating a higher PP (actual watts) or your sprint potential under fatigue (estimated PP).
For details see this article – Breaking Through the Xert Way! – Xert (baronbiosys.com)
Your fitness signature will also fluctuate without BTs based upon changes in TL (Training Load: volume and intensity) and lack of training (decay method).
For Difficulty explanation see this article – Difficulty Score – Xert (baronbiosys.com)
Also check out my Onboarding post – Onboarding steps for Xert trial users and newbies - General - Xert Community Forum (xertonline.com)