If you link directly to the full URL (including the instance), you'll take anyone who clicks it to that instance, and they won't be logged in. This is usually not what you want. Example: https://pawb.social/c/tech - This link will take you to my instance.
If you remove the instance URL, and just leave /c/communityname@instance - for example, /c/tech@pawb.social - the link will still take you to the community, but you'll still be on your instance. This is usually desirable.
Basically, instance -> community = link to that instance. Community -> instance = link to the community in whatever instance the user clicks it in.
You can also use ! instead of /c/ - I think this might work better for Kbin users (since they use /m/ instead of /c/ - can't verify this). In that case, it'd be: !tech@pawb.social