TP serves a similar purpose as FTP of course, but it isn’t derived in the same manner.
The numbers closely align for many users, but not always.
The critical point is whether you can successfully train with your current signature values.
Threshold Power is a parameter of your Fitness Signature that represents the highest power that can be sustained without accumulating short-term fatigue – e.g. without MPA decreasing.
FTP is typically derived (estimated) from a 20-minute, RAMP, or KM test.
TP is derived by analyzing your activities including actual power measured and theoretical power as determined by your maximal efforts under fatigue (MPA drawn down).
To validate your fitness signature using Xert’s methodology you can select one of the fitness test workouts from the library and ride it in Slope mode (if indoors) exceeding targets if able plus ride to failure pts where indicated.
See Xert Fitness Tests: From old trainer to smart trainer. Set FTP - Training - Xert Community Forum
Fitness signature values float up and down day to day. You can see this in action by viewing your Activities Table. Bumps will occur if BTs are detected. Decline will occur (due to decay) if there are long periods between BTs without increases in TL. A Near BT will also lower values a bit on purpose.
See: Breaking Through the Xert Way! – Xert
While some content is outdated you should find my newbie tips of value –
Onboarding steps for Xert trial users and newbies - General - Xert Community Forum