How to get the most out of FB Plus?

Hi! I've been working out with FB for years but just joined Plus. So far, what I like most is the access to the programs that are 3 days per week. I am a bit confused about routines and custom workouts though. I really can't figure out what they are for and why I should use them. So, if I go to a Pilates studio Tuesday/Thurs/Saturday, would I enter that in routines or in custom, to just have a record of it? Or are these features for creating your own workout program rather than following a pre-designed one? I've searched the site and still don't understand.

I'm totally fine paying for FB Plus just to support the community and make the free things available for others who can't afford to join, but I'm just wondering if I can be getting more out of it.

Thanks, and happy spring! I am feeling super awesome today because I'm on week 2 of Blend and loving it.