Hey again @sundles
Yes, the "Purge" feature within NI Kontakt is intended to work dynamically on your active session, it's just to save RAM & processing power by unloading the Kontakt Instrument samples that you're not using in this session from memory.
So when you load a new instrument in Kontakt you can see the amount of RAM used by all samples in the "Memory" field, if you use the "Purge All Samples" option, you will see that it falls down to zero. Now if you play any new Midi notes on a controller or if you just playback your Midi sequence from your DAW, Kontakt will only load those samples into memory, therefore saving RAM.
This won't affect the Kontakt instrument library, so no worries for other projects. Even loading the same instrument again in your instance of Kontakt or in a new one will load all the samples again in fact, and you'll need to purge this second instance of the instruments too if needed.
It's also good to know that the Purge options are not only usable per Kontakt instrument but for a all rack as well, click on the "New Instrument" menu ( the Floppy Disk Icon ) and you should find a "Global Purge" Menu that will offer the same options for the all Kontakt rack.
Hope that helps Cheers !