TP is analogous to FTP for training purposes but they are derived in a different manner.
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.
LTP represents the lowest possible value for Threshold Power when all Endurance Energy reserves have been depleted:
Here’s a good discussion about using LTP when training –
LTP questions - Support - Xert Community Forum (xertonline.com)
For many users LTP is close to their LT1. If not, RPE and HR will indicate how your LT1 compares.
You can reference LTP as a ceiling in watts you want to manage during easy Z2 endurance rides.
Here’s my favorite over/under LTP indoor workout: (3) Xert - Workout Details (xertonline.com)
Reference: