ARDR is a forging token of the parent chain of the Ardor platform. ARDR is a locomotive, an engine that pulls the "cars" (child chains). Can't do without an engine, it's very valuable. Only the parent chain and Ardor token can do forging.
IGNIS is one of the child chains. It is the first default community child chain, it very likely will be the most decentralized of all child chains. These characteristics give it value.
Users of all child chains will have to pay fees to have their "cars pulled" (transactions processed into blocks) to users who own Ardor tokens and do forging (act as the engine).
The more child chains the platform will host and the more activity within those child chains, the more demand for Ardor tokens and the more profit to forgers.
Private and public child chains can be ordered to be added to the platform by companies, communities, towns, etc. They will all have to pay fees to forgers to have their transactions processed. This locomotive can pull many "cars". I hope this helps to understand the Ardor design better.
Thanks for that explanation!
