There are four ways to store data:

1. on a port: has obvious downsides. It will be per execution, and the value does get stored in the port, but you'd have to mangle it and bleh.
2. static internal member to the class: shared by all copies everywhere
3. internal member to the class: shared by all uses of the instance of the unit
4. in the Unit's data (an optional system that you won't have seen unless you look in a unit that uses it). This is stored per use.