Can I use “planet diffs” if I only have data of 2 countries?

I'm currently running my own nominatim server with data of just a couple of countries. Nominatim supports automatic updates, and it uses the "planet diffs" by default, according to their manual.

Is it any problem that it uses all diffs of the planet if it only hasthe data of a couple of countries? Will it somehow add the diffs of like Australia while I don't have any data of Australia to begin with?

If that's not possible, how can I setup Nominatim to only listen to diffs of the countries it has the data of?