VW recommends charging the battery only to 80% if possible. Fully charging a Li-Ion battery is stressing it (aka battery degradation) and shortens its useful life. Plenty of material on the net if you want to dig deeper into why and how that is the case.
A decent and practical approach is this:
- On regular days charge to 80%, drive your daily errands, and charge back up to 80% when you need it.
- Before a long trip where you want the full range, charge to 100% so your first leg of the trip can be maximized.
Battery reserve, the way I understand it, is the 5% at the bottom and 4% at the top that VW makes unavailable. (77 kWh usable, 84 kWh theoretical for the large battery).