Hello Laura
Additive means that the old licenses remain and you simply add the new ones, so no need to uninstall, in general yes, they are additive, but some of them are not, it actually depends on how the key is generated and the PC version you have, for your point of view all of then should be additive and your old key must remian if you add a new one, there are few specific situation when you get a non additive license, but the most of times this is because is required by support.
let me know if you have further questions.