Please note that what I am talking about here is traditional materials, materials used in the making of fire with Flint, Steel & Tinderbox. ( I am NOT in any way referring to materials used in the making of fire using the modern ferocerium rod ).
Tinder is a plant material that will catch & hold a spark from flint & steel. It can also be used to transfer an ember from the fire-board of a fire-bow. The dust created during the function of a fire-bow is also tinder.
Kindling is the material that actually takes fire when brought in contact with smouldering tinder. Dried grass, teased Stringybark bark, coconut fibre, teased natural rope, a birds nest made of fine natural materials often including the aforementioned plant materials. Kindling is also small twigs & sticks used in the initial making of fire.
The so called “tinder nest” is NOT tinder in the true sense of the word, it is kindling. Fat wood is NOT tinder in the true sense of the word, it is kindling. Neither of these plant materials will catch & hold a spark struck from flint & steel.