Hello, we differentiate between them by geographic boundaries and also by Charity Commission/ NGO Regulator classifications.
International NGO - an NGO that operates externally to its country of registration.
So for example, World Vision International would be an International NGO even though it is based in Kenya (this is also Charity Commission/country regulator classification).
National NGO - an NGO that is both registered in and operates in one country. So for example World Vision Nepal (this is also Charity Commission/country regulator classification).
Regional NGO - an NGO whose activities are specific to a particular region but may be registered in one country. An example of this would be some of the Asia-Pacific regional networks (can't think of a name off hand). This one has always been the hardest to interpret.
If you want to change the meaning of these three categories, then I'd suggest you replicate the wording of the DAC codes exactly and lose particularly the words regional and national - to a development practitioner or journalist, there's no way that they would interpret Regional NGO as 'Developing country-based NGO'.
Also if you are intending to change the meaning of the codes, you will need to have some kind of way for data users to interpret the meaning prior to the change, as it's unlikely that NGO publishers will recode their historical data.