Well, Uplay is ubisoft's Digital Distribution Platform, much like Valve's 'Steam' and EA's 'Origin'
Purchased games are tied to individual accounts on these platforms through the use of unique keys, these keys can come with boxed retail games, or through online retail sellers.
CD Key, Steam Key, Origin Key, Uplay key, it's all the same game, just on different distribution platforms.
A DVD boxed version of SH5 requires you to install uplay, as it's the publishers choice of distribution platform. You install uplay, enter your DVD key and tie the game to your uplay account, and from then on launch the game through the uplay launcher.
I already had a uplay account and launcher installed from other ubisoft games I own, so I just bought a silent hunter 5 key from an online reseller, and activated it on my uplay account.
There are literally hundreds of online retailers selling digital keys for games on Steam, Uplay and Origin. Never buy direct from the publisher, you can nearly always find it cheaper elsewhere, and the older the game, the cheaper you can get it (that goes for Steam too)
An an example, Sniper Elite 3 just came out a month ago, it's currently £39.99 GBP to purchase through Steam, the full publishers release price.
G2play will sell you a steam key for £14.87
http://www.g2play.net/category/7468/...iii-steam-key/
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