You make a custom comparison and manually put the times in on Livesplit.

On Livesplit right click and go to Edit Splits, then you'll see the highlighted option from my screenshot. After that you manually input times to the split editor

It's a custom comparison they made using a website like amyy.me/splitmaker/ . The idea is that you upload your splits file which has your best segment times, and then it distributes total timesave for a given target time roughly according to split length (i.e. longer splits are given more timesave than the shorter splits).

For example, if you're going for subhour you could set the target time on that splitmaker to be 0:59:59.99, and it would calculate how much off sum of best you need to play (e.g. if sob = 58:59, then you have 60 seconds to lose and still get 59:59) and then distribute that time over each split and save it as a comparison so you can see how far ahead/behind it you are during a run like you would for your PB. Another similar use for it is putting in your current PB as a target time and it distributes your timeloss more evenly across the run, making what some call a "balanced PB" comparison (because you may have a lot of timesave in one split and it can be easier to compare to a comparison where that time is spread out over the run)

I hope that makes sense, it's a bit tricky to just explain so hopefully I got the idea across.