The Variable type in Global Variables can be used two ways, depending on whether you plan to change its value while the form runs:
Both use the exact same Variable type — there's no separate "constant" option to pick. The difference is simply whether you leave a Default value as the value's only source of truth, or let other parts of the form update it later.



The At run time panel on the right confirms exactly what will happen — for example, "Starts as: High" — so you can see the effect before saving.
If you don't reference this variable anywhere that changes it (no workflow node writes to it, no field lets someone edit it directly), it behaves like a constant: the Default value you set is the value it keeps everywhere it's used, for the life of the form. This is the simplest way to define something like a company name, a fixed tax rate, or standard wording that shows up in several labels — set it once here, and update it in one place if it ever needs to change.
If you do want the value to change — for example, a workflow node calculates a total and stores it here, or a field lets the user edit it — just leave the variable unlocked as normal. Any node or field with access to the form can read the current value or write a new one, and everywhere else that references the variable will reflect the latest value.