But as Oxa said, only a user variable can hold a trigger expression as a value; a constant can't.
That's right, when you're adding constants through the "Add Constant", you can't put a formula or a trigger variable in the value field.
But you can change the value of the constant through a trigger, and in that trigger you can assign it all sorts of trigger variables and formulae.
To give you a simple example. When you program a staking plan, you want to create a constant which will hold the size of the initial stake, for instance
init_bet. It is normally a number of your currency units, e.g. 2.
But then you may need a variable that holds the value of the current bet size. This variable can change from market to market, depending on the previous loss. Also, this variable's initial value must be equal to the constant that denominates the initial stake. So you create a trigger that "sets user variable", let's say,
current_bet, and its value will be equal to
init_bet.
This is an example of when you need specifically a trigger variable, and not a constant. The variable
current_bet is initialised with a constant, and so it can't be a constant itself.
I pray that you did understand my ramblings!