When you create or edit a link you can tag it with the subreddit you plan to post it in (for example r/SaaS). The tag is a label for your own analytics; it does not change how the link behaves.
With links tagged, your dashboard shows a Top subreddits ranking: total clicks per community across all your links. That tells you which subreddits are worth more of your time.
A common setup: create one link per subreddit for the same destination, tag each accordingly, and compare.