I'm not trained by lash perfect but I think I can explain this. Individual extensions you will be isolating each and every natural lash and securing an extension to it, that is why it is so time consuming. These are for long term use and after 2-4 weeks an infill will be required where you isolate any new natural lash/NL that has lost its extension with a new extension. These can keep being infilled indefinitely.
Lash express, is express lashes where no isolation of the natural lash takes place and the extensions are glued inbetween and on top of the natural lash. This is why it's such a quick treatment. Due to the extensions being placed on non isolated lashes, they could be joined to multiple natural lashes. This means they will need to be removed after 2-3 weeks and cannot be infilled. So these are for short term/occasion use.
Hope that makes sense x