This value available under Account Settings / Profile, represents the total Training Load that you had at the time of your very first activity on the system. Since there is no data previous to this, you should adjust this value to be what your Training Load was most likely to have been at that time. If you’re not sure, enable Calculate an Estimate to allow the system to calculate one for you. The system will use your average daily XSS for your entire activity history as an estimate of your training load.
Changing the value will affect the relationship between Training Loads and your Fitness Signature variables at the beginning of your progression history in the system.