Well don't you find that
nice-keywords-you-can-choose-in-acp-fxx.html is a lot more SEO friendly than
viewforum.php?f=xx ?
The -fxx purpose is just to pass the forum ID to phpBB.
But, if you want to go further into RTFM, you'll notice that there is a "remove forum id" option in acp which does exactly that, eg get rid of the -fxx bit with no code changes

Once you activate the option, you'll have to set custom URLs in forum url management before the -fxx actually goes away. Please take good care about the fact that forum url without ID will NOT be automatically redirected if you later change the forum URL by the zero duplicate (as it would be the case if you kept them). It's not really an issue since you're not supposed to change forum URL that often and also because you can still update forum titles without altering forum urls, but this means that you should better be sure about what you want to do when you set the custom forum urls without id.
I encourage you to do some local testing in order to better understand the numerous possibilities offered by the mod, and elect the one that best fit your case without jeopardizing your live forum indexing and ranking.
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