U can have a copy of 2015 client from here.
https://rathena.org/board/topic/104205-2015-client-support
The clients which got released to be easy understanding is not yet patched. That is why u will need nemo patcher to patch that client so that it can be use to login into the server u creates earlier.
Diff is nemo patches list. Before nemo exist, in order to prepare the client, we need to diff it.. by nowadays, just patch it using nemo patcher.. however the word 'diff' still stick until today as that is the process to prepare the client.
How to diff / patch the client using nemo patcher? U can go to nemo patcher topic. The person who develop the nemo patcher already provide the guide how to diff / patch the client.
And if u looking for older clients, u can visit this link.
http://supportmii.com/ro1/Clients/
