With my quick testing of the Xtar VC8 charger, these are my observations:

Assuming using a USB power source with QC2/9v output:
- only 1 slot populated, max charge rate is 3A (only slot 1 & 8 support this, but only 1 of them can charge at 3A at a time),
- only 2 slots populated, max charge rate is 2A
- 3-4 slots populated, max charge rate is 1A
- 5-8 slots populated, max charge rate is 0.5A

-user selectable charge current (3A/2A/1A/0.5A/0.25A), subject to above
-slots 1-4 are linked (select 1 charge rate) and slots 5-8 are linked (can select another charge rate)

Type-C to type-C cable with type-C charger doesn't work as is, as per my test (similar to the LT1 lantern USB-C port)