The distinction is that, for example, a 'world foods' (aka 'ethnic'!) aisle in a supermarket categorises stuff efficiently that the local ethnic minority group (predominantly) may be interested in, while more widely consumed (or consumed by local ethnic majority group) foods are distributed elsewhere categorised more according to food type; compared to saying 'you are {local ethnic minority}ian so you are not allowed to purchase {local ethnic majority} foods', which would be discrimination.
Whether it applies to the example above though is I think more debatable.
r/lesbians is a porn subreddit with content restricted to female same sex porn content. This is just categorisation.
r/truelesbians was supposed to a subreddit for lesbians to aggregate and discuss stuff. What ended up happening was while it was still a lesbian focused subreddit, it became a breeding ground for anti-trans/TERF content/discussions/rhetoric and the moderation of the sub enforced those beliefs. This is what makes r/truelesbians discriminatory instead of just being categorisation. It takes a step from "don't post content that doesn't belong here" to "don't post content that doesn't belong here AND don't post at all here if you match some arbitrary biological definition we don't like or if you support those people".
You also have r/actuallesbians which branched off from r/truelesbians when they got TERFy. r/actuallesbians is inclusive of all people who identify as female but have the same original focus of providing a platform to discuss and aggregate female same sex/lesbian content/discussions. This once again is categorisation and not discrimination because anyone can post there as long as the posts are on topic. Additionally the content of the sub isn't discriminatory in this case because it's focused on a topic (about same sex female relationships) rather than focused against another (against trans individuals).
So r/lesbians and r/actuallesbians both are categorical while r/truelesbians was discriminatory on top of being categorical because of the exclusionary nature of the subreddit.
