The value can be changed manually by changing it in the constants, saving the trigger and then reloading the trigger.
The value can also be changed by trigger conditions like you mention, so you can add to it, or reset it to 0 depending on the outcome of a race or whatever trigger condition you choose.
The value will retain its' last value when the programme is exited and will reload with that value when the programme is restarted if 'remember last value' is selected next to the constant.
This is how I understand that it works, and how it seems to work in my experience.
Hope that helps